feat: full tool call support (OpenAI + Gemini endpoints)

- store.rs: Add tool context storage (active tools, tool config, pending
  tool results, call_id mapping, last function calls for history rewrite)
- types.rs: Add tools/tool_choice fields to ResponsesRequest, add
  build_function_call_output helper for OpenAI function_call output items
- modify.rs: Replace hardcoded get_weather with dynamic ToolContext
  injection. Add openai_tools_to_gemini and openai_tool_choice_to_gemini
  converters. Add conversation history rewriting for tool result turns
  (replaces fake 'Tool call completed' model turn with real functionCall,
  injects functionResponse before last user turn)
- proxy.rs: Build ToolContext from MitmStore before calling modify_request.
  Save last_function_calls for history rewriting on subsequent turns
- responses.rs: Store client tools in MitmStore before LS call. Detect
  function_call_output in input array for tool result submission. Return
  captured functionCalls as OpenAI function_call output items with
  generated call_ids and stringified arguments
- gemini.rs: New Gemini-native endpoint (POST /v1/gemini) with zero
  format translation. Accepts functionDeclarations directly, returns
  functionCall in Gemini format directly
- mod.rs: Wire /v1/gemini route, bump version to 3.3.0

src/api/mod.rs

@@ -1,6 +1,7 @@
 //! Axum API server — OpenAI-compatible Responses + Chat Completions endpoints.
 
 mod completions;
+mod gemini;
 mod models;
 mod polling;
 mod responses;
@@ -41,6 +42,7 @@ pub fn router(state: Arc<AppState>) -> Router {
             "/v1/chat/completions",
             post(completions::handle_completions),
         )
+        .route("/v1/gemini", post(gemini::handle_gemini))
         .route("/v1/models", get(handle_models))
         .route("/v1/sessions", get(handle_list_sessions))
         .route("/v1/sessions/{id}", delete(handle_delete_session))
@@ -59,11 +61,12 @@ pub fn router(state: Arc<AppState>) -> Router {
 async fn handle_root() -> Json<serde_json::Value> {
     Json(serde_json::json!({
         "service": "antigravity-openai-proxy",
-        "version": "3.2.0",
+        "version": "3.3.0",
         "runtime": "rust",
         "endpoints": [
             "/v1/chat/completions",
             "/v1/responses",
+            "/v1/gemini",
             "/v1/models",
             "/v1/sessions",
             "/v1/token",