feat(auth): add OAuth2 support for xAI with PKCE and token persistence

- Implemented xAI OAuth2 integration with PKCE (Proof Key for Code Exchange) support.
- Added logic for token exchange, refresh, and persistent storage in JSON format.
- Created `xai` package with helpers for OAuth discovery, API token handling, and URL building.
- Introduced `XAIExecutor` for integrating xAI credentials into runtime HTTP requests.
- Added unit tests to validate OAuth flow, token persistence, and endpoint validation.

internal/cmd/auth_manager.go

@@ -6,7 +6,7 @@ import (
 
 // newAuthManager creates a new authentication manager instance with all supported
 // authenticators and a file-based token store. It initializes authenticators for
-// Gemini, Codex, Claude, Antigravity, and Kimi providers.
+// Gemini, Codex, Claude, Antigravity, Kimi, and xAI providers.
 //
 // Returns:
 //   - *sdkAuth.Manager: A configured authentication manager instance
@@ -18,6 +18,7 @@ func newAuthManager() *sdkAuth.Manager {
 		sdkAuth.NewClaudeAuthenticator(),
 		sdkAuth.NewAntigravityAuthenticator(),
 		sdkAuth.NewKimiAuthenticator(),
+		sdkAuth.NewXAIAuthenticator(),
 	)
 	return manager
 }