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)

src/app.rs

@@ -683,10 +683,14 @@ impl AppBuilder {
         self.init_database().await?;
         self.init_secrets().await?;
 
-        // Post-init validation: if a non-nearai backend was selected but
-        // credentials were never resolved (deferred resolution found no keys),
-        // fail early with a clear error instead of a confusing runtime failure.
-        if self.config.llm.backend != "nearai" && self.config.llm.provider.is_none() {
+        // Post-init validation: backends with dedicated config (nearai, gemini_oauth,
+        // bedrock) handle their own credential resolution. For registry-based backends,
+        // fail early if no provider config was resolved.
+        if self.config.llm.backend != "nearai"
+            && self.config.llm.backend != "gemini_oauth"
+            && self.config.llm.backend != "bedrock"
+            && self.config.llm.provider.is_none()
+        {
             let backend = &self.config.llm.backend;
             anyhow::bail!(
                 "LLM_BACKEND={backend} is configured but no credentials were found. \