feat(gemini_oauth): full Gemini CLI OAuth integration with Cloud Code API (#1356)

* feat: integrate Gemini CLI OAuth with Cloud Code API

- Add gemini_oauth.rs: full OAuth flow with PKCE, token refresh,
  and Cloud Code project discovery (loadCodeAssist + onboardUser)
- Route preview/gemini-3 models through cloudcode-pa.googleapis.com
  with proper project ID injection in request payload
- Trigger OAuth login during onboarding wizard (not first chat message)
- Support manual redirect URL paste as fallback (tokio::select race)
- Parse 429 rate-limit errors with retry_after from Google response
- Add static model list: gemini-1.5/2.0/2.5/3.0/3.1 variants
- Add GeminiOauthConfig with default credentials path (~/.gemini/)

* feat(gemini): implement function calling, generationConfig, and update models

- Implement function calling support (functionDeclarations, functionResponse)
- Add functionCall SSE parsing and empty stream retry support
- Add generationConfig (temperature, maxOutputTokens)
- Add thinkingConfig for Gemini 3 and thinking models
- Add toolConfig (functionCallingConfig.mode)
- Fix .expect() panics with .ok_or_else()
- Restrict oauth credentials file permissions to 0600
- Update docs and FEATURE_PARITY.md
- Update wizard to current Gemini 3.1 and 2.5 models

* fix: address code review issues in gemini-cli OAuth integration

- Add cache_read_input_tokens/cache_creation_input_tokens fields (value 0)
- Implement manual Debug for OAuthCredential to redact tokens
- Fix hardcoded /tmp: use GeminiOauthConfig::default_credentials_path()
- Replace emoji output with plain text markers
- Propagate Client::builder() errors instead of silent fallback
- Use tokio::fs for all file I/O in CredentialManager (was std::fs)
- Use if let Some(ref pid) to avoid consuming credential.project_id
- Extract uses_cloud_code_api() helper; route by major version (gemini-2+)
- Concatenate multiple system messages into systemInstruction
- Include functionCall parts in assistant message conversion
- Add 401 retry loop with allow_retry flag for auth failures
- Remove biased from tokio::select! in OAuth callback handler
- Remove hardcoded context_length 1M; vary by model family
- Change GOOG_API_CLIENT from Node.js spoof to gl-rust/1.0.0
- Implement list_models() with static model list
- Move create_gemini_oauth_provider() before test module (clippy)
- Fix 9 additional clippy warnings (collapsible_if, map_or, needless_borrow)
- Run cargo fmt

* Add dedicated regression tests for Gemini OAuth fixes

* style: fix formatting in Gemini OAuth regression tests

* feat(gemini-oauth): implement code review v3 refinements

- Add force_refresh() for 401 retry (bypass timestamp check)
- Standardize Gemini model list across docs, wizard, and provider
- Restore gemini-3 check for thinkingConfig
- Redact sensitive tokens in GoogleTokenRefreshResponse Debug output
- Use dynamic version for GOOG_API_CLIENT
- Improve model_metadata() context length heuristics
- Use strip_prefix("data:") for safer SSE parsing
- Skip re-auth in wizard if keeping existing provider

* feat(gemini_oauth): full Cloud Code API integration with project discovery

- Register gemini_oauth as a dedicated backend in config/llm.rs (skip
  registry fallback, preserve backend name, suppress unknown-backend warning)
- Fix app.rs credential guard to exclude backends with dedicated configs
  (gemini_oauth, bedrock) from the provider.is_none() check
- Auto-discover Cloud Code project_id via loadCodeAssist when credentials
  lack it (e.g. created by the original Gemini CLI)
- Persist discovered project_id to credentials file for subsequent runs
- Add safety settings (BLOCK_NONE), gated behind GEMINI_SAFETY_BLOCK_NONE env
- Add thinkingConfig: budget-based for Gemini 2.5, level-based for Gemini 3.x
  (without includeThoughts to avoid empty responses from reasoning.rs stripping)
- Add thought signature injection for Gemini 3.x preview APIs
- Add history curation to filter invalid model outputs before re-sending
- Add extended generationConfig env vars (topP, topK, seed, penalties,
  responseMimeType, responseJsonSchema, cachedContent)
- Add custom headers support via GEMINI_CLI_CUSTOM_HEADERS
- Add API key auth mode (GEMINI_API_KEY + GEMINI_API_KEY_AUTH_MECHANISM)
- Add SSE metadata extraction (modelVersion, credits, promptFeedback,
  groundingMetadata, citationMetadata, cachedContentTokenCount)
- Add countTokens API support
- Add new models to wizard (gemini-3.1-pro-preview-customtools,
  gemini-3-pro-preview, gemini-3.1-flash-lite-preview)
- Update docs/LLM_PROVIDERS.md with new models and routing rules
- Rewrite regression tests with comprehensive coverage (23 unit tests pass)

* fix: CI violations — add safety comment on expect, fix fmt

- Add '// safety: hardcoded literal' to regex .expect() to satisfy
  the no-panic-in-prod CI check
- Fix cargo fmt whitespace in collapsible if-let chain

* fix: address PR review feedback from gemini-code-assist

- Fix parse_custom_headers to preserve commas in values by splitting
  only on commas followed by a header-name:colon pattern (manual scan
  instead of simple split(','))
- Use matches! macro for backend exclusion check in app.rs
- Merge SSE metadata extraction into single pass (was iterating twice)
- Replace fragile substring-based context_length with explicit match
  on known Gemini model IDs via gemini_context_length()
- Add missing models to regression test (8 models, not 5)

* fix: address Copilot PR review feedback

- Fix empty text part for assistant messages with tool calls
  (curate_contents could drop entire model turn)
- Propagate cache_read/creation_input_tokens in complete_with_tools
- Log warning on save_credential failure instead of silently ignoring
- Fix doc comment to mention underscore in header name pattern
- Handle gemini-oauth (hyphen variant) in setup wizard display
- Fix docs: thinkingConfig uses thinkingBudget/thinkingLevel, not
  includeThoughts

* fix: add missing allow_always field after staging merge

* fix(gemini_oauth): align header parser doc with implementation [skip-regression-check]

Update parse_custom_headers doc comments to include underscore in the
header-name character class, matching the actual implementation.
Also fix formatting from merge.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix(gemini_oauth): curate_contents per-part filtering and dead code removal

Fix curate_contents to filter invalid parts individually instead of
dropping entire model turn sequences. Previously a single empty text
part would discard all consecutive model turns including valid
functionCall parts, breaking the tool-call flow.

Also remove unused MID_STREAM_* constants.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* style(gemini_oauth): rustfmt formatting [skip-regression-check]

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix(llm): support smart routing cheap model for gemini_oauth backend

Add explicit gemini_oauth handling in create_cheap_provider_for_backend()
to create a GeminiOauthProvider with the cheap model swapped in. Without
this, setting LLM_CHEAP_MODEL with gemini_oauth backend would fail with
a confusing "no registry provider config available" error.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* docs: add Gemini OAuth env vars to .env.example [skip-regression-check]

Document GEMINI_MODEL, GEMINI_CREDENTIALS_PATH, GEMINI_API_KEY, and
all extended generation config env vars in the example config file.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: [email protected] <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

.env.example

@@ -4,7 +4,7 @@ DATABASE_POOL_SIZE=10
 
 # LLM Provider
 # LLM_BACKEND=nearai           # default
-# Possible values: nearai, ollama, openai_compatible, openai, anthropic, github_copilot, tinfoil, openai_codex
+# Possible values: nearai, ollama, openai_compatible, openai, anthropic, github_copilot, tinfoil, openai_codex, gemini_oauth
 # LLM_REQUEST_TIMEOUT_SECS=120  # Increase for local LLMs (Ollama, vLLM, LM Studio)
 
 # === Anthropic Direct ===
@@ -110,6 +110,23 @@ NEARAI_AUTH_URL=https://private.near.ai
 # OPENAI_CODEX_AUTH_URL=https://auth.openai.com  # override (rare)
 # OPENAI_CODEX_API_URL=https://chatgpt.com/backend-api/codex  # override (rare)
 
+# === Google Gemini (OAuth, Gemini CLI compatible) ===
+# LLM_BACKEND=gemini_oauth
+# GEMINI_MODEL=gemini-2.5-flash                  # default
+# GEMINI_CREDENTIALS_PATH=~/.gemini/oauth_creds.json  # default
+# GEMINI_API_KEY=...                             # optional: use API key instead of OAuth
+# GEMINI_API_KEY_AUTH_MECHANISM=query             # "query" (default) or "header"
+# GEMINI_SAFETY_BLOCK_NONE=true                  # disable safety filters (default: false)
+# GEMINI_CLI_CUSTOM_HEADERS=Key:Value,Key2:Value2
+# GEMINI_TOP_P=0.95
+# GEMINI_TOP_K=40
+# GEMINI_SEED=42
+# GEMINI_PRESENCE_PENALTY=0.0
+# GEMINI_FREQUENCY_PENALTY=0.0
+# GEMINI_RESPONSE_MIME_TYPE=application/json
+# GEMINI_RESPONSE_JSON_SCHEMA={"type":"object"}
+# GEMINI_CACHED_CONTENT=cachedContents/abc123
+
 # For full provider setup guide see docs/LLM_PROVIDERS.md
 
 # Channel Configuration