**feat(auth, executor, cmd): add Antigravity provider integration**

- Implemented OAuth login flow for the Antigravity provider in `auth/antigravity.go`.
- Added `AntigravityExecutor` for handling requests and streaming via Antigravity APIs.
- Created `antigravity_login.go` command for triggering Antigravity authentication.
- Introduced OpenAI-to-Antigravity translation logic in `translator/antigravity/openai/chat-completions`.

**refactor(translator, executor): update Gemini CLI response translation and add Antigravity payload customization**

- Renamed Gemini CLI translation methods to align with response handling (`ConvertGeminiCliResponseToGemini` and `ConvertGeminiCliResponseToGeminiNonStream`).
- Updated `init.go` to reflect these method changes.
- Introduced `geminiToAntigravity` function to embed metadata (`model`, `userAgent`, `project`, etc.) into Antigravity payloads.
- Added random project, request, and session ID generators for enhanced tracking.
- Streamlined `buildRequest` to use `geminiToAntigravity` transformation before request execution.

sdk/cliproxy/service.go

@@ -333,6 +333,8 @@ func (s *Service) ensureExecutorsForAuth(a *coreauth.Auth) {
 			s.coreManager.RegisterExecutor(executor.NewAIStudioExecutor(s.cfg, a.ID, s.wsGateway))
 		}
 		return
+	case "antigravity":
+		s.coreManager.RegisterExecutor(executor.NewAntigravityExecutor(s.cfg))
 	case "claude":
 		s.coreManager.RegisterExecutor(executor.NewClaudeExecutor(s.cfg))
 	case "codex":
@@ -634,6 +636,10 @@ func (s *Service) registerModelsForAuth(a *coreauth.Auth) {
 		models = registry.GetGeminiCLIModels()
 	case "aistudio":
 		models = registry.GetAIStudioModels()
+	case "antigravity":
+		ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+		models = executor.FetchAntigravityModels(ctx, a, s.cfg)
+		cancel()
 	case "claude":
 		models = registry.GetClaudeModels()
 		if entry := s.resolveConfigClaudeKey(a); entry != nil && len(entry.Models) > 0 {