feat(proxy): wire Gemini Native format into proxy core and Claude adapter

Integrate gemini_native api_format throughout the proxy pipeline:
- ClaudeAdapter: detect Gemini provider type, Google/GoogleOAuth auth
  strategies, and suppress Anthropic-specific headers for Gemini targets
- Forwarder: Gemini URL resolution, shadow store threading, endpoint
  rewriting to models/*:generateContent with stream/non-stream variants
- Handlers: route Gemini streaming through streaming_gemini adapter and
  non-streaming through transform_gemini converter
- Server/State: add GeminiShadowStore to shared ProxyState
- StreamCheck: support gemini_native health check with proper auth headers

src-tauri/src/proxy/handler_context.rs

@@ -218,9 +218,11 @@ impl RequestContext {
             non_streaming_timeout,
             state.status.clone(),
             state.current_providers.clone(),
+            state.gemini_shadow.clone(),
             state.failover_manager.clone(),
             state.app_handle.clone(),
             self.current_provider_id.clone(),
+            self.session_id.clone(),
             first_byte_timeout,
             idle_timeout,
             self.rectifier_config.clone(),