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/interfaces.go

@@ -14,10 +14,11 @@ var ErrRefreshNotSupported = errors.New("cliproxy auth: refresh not supported")
 // LoginOptions captures generic knobs shared across authenticators.
 // Provider-specific logic can inspect Metadata for extra parameters.
 type LoginOptions struct {
-	NoBrowser bool
-	ProjectID string
-	Metadata  map[string]string
-	Prompt    func(prompt string) (string, error)
+	NoBrowser    bool
+	ProjectID    string
+	CallbackPort int
+	Metadata     map[string]string
+	Prompt       func(prompt string) (string, error)
 }
 
 // Authenticator manages login and optional refresh flows for a provider.