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/setup/wizard.rs

@@ -1708,10 +1708,22 @@ impl SetupWizard {
                         "gemini-3.1-pro-preview".into(),
                         "Gemini 3.1 Pro (Latest, strongest reasoning)".into(),
                     ),
+                    (
+                        "gemini-3.1-pro-preview-customtools".into(),
+                        "Gemini 3.1 Pro Custom Tools (Enhanced tool use)".into(),
+                    ),
+                    (
+                        "gemini-3-pro-preview".into(),
+                        "Gemini 3 Pro (Preview)".into(),
+                    ),
                     (
                         "gemini-3-flash-preview".into(),
                         "Gemini 3 Flash (Fast preview with thinking)".into(),
                     ),
+                    (
+                        "gemini-3.1-flash-lite-preview".into(),
+                        "Gemini 3.1 Flash Lite (Preview, lightweight)".into(),
+                    ),
                     (
                         "gemini-2.5-pro".into(),
                         "Gemini 2.5 Pro (Stable, strong reasoning)".into(),