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/config/llm.rs

@@ -10,7 +10,6 @@ use crate::llm::registry::{ProviderProtocol, ProviderRegistry};
 use crate::llm::session::SessionConfig;
 use crate::settings::Settings;
 
-
 impl LlmConfig {
     /// Create a test-friendly config without reading env vars.
     #[cfg(feature = "libsql")]
@@ -75,8 +74,10 @@ impl LlmConfig {
             backend_lower == "nearai" || backend_lower == "near_ai" || backend_lower == "near";
         let is_bedrock =
             backend_lower == "bedrock" || backend_lower == "aws_bedrock" || backend_lower == "aws";
+        let is_gemini_oauth = backend_lower == "gemini_oauth" || backend_lower == "gemini-oauth";
 
-        if !is_nearai && !is_bedrock && registry.find(&backend_lower).is_none() {
+        if !is_nearai && !is_bedrock && !is_gemini_oauth && registry.find(&backend_lower).is_none()
+        {
             tracing::warn!(
                 "Unknown LLM backend '{}'. Will attempt as openai_compatible fallback.",
                 backend
@@ -124,7 +125,7 @@ impl LlmConfig {
         };
 
         // Resolve registry provider config (for non-NearAI, non-Bedrock backends)
-        let provider = if is_nearai || is_bedrock {
+        let provider = if is_nearai || is_bedrock || is_gemini_oauth {
             None
         } else {
             Some(Self::resolve_registry_provider(
@@ -199,6 +200,8 @@ impl LlmConfig {
                 "nearai".to_string()
             } else if is_bedrock {
                 "bedrock".to_string()
+            } else if is_gemini_oauth {
+                "gemini_oauth".to_string()
             } else if let Some(ref p) = provider {
                 p.provider_id.clone()
             } else {