Fixed: #499 #985

feat(oauth): add support for customizable OAuth callback ports

- Introduced `oauth-callback-port` flag to override default callback ports.
- Updated SDK and login flows for `iflow`, `gemini`, `antigravity`, `codex`, `claude`, and `openai` to respect configurable callback ports.
- Refactored internal OAuth servers to dynamically assign ports based on the provided options.
- Revised tests and documentation to reflect the new flag and behavior.

sdk/auth/gemini.go

@@ -45,8 +45,9 @@ func (a *GeminiAuthenticator) Login(ctx context.Context, cfg *config.Config, opt
 
 	geminiAuth := gemini.NewGeminiAuth()
 	_, err := geminiAuth.GetAuthenticatedClient(ctx, &ts, cfg, &gemini.WebLoginOptions{
-		NoBrowser: opts.NoBrowser,
-		Prompt:    opts.Prompt,
+		NoBrowser:    opts.NoBrowser,
+		CallbackPort: opts.CallbackPort,
+		Prompt:       opts.Prompt,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("gemini authentication failed: %w", err)