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.

sdk/auth/refresh_registry.go

@@ -13,6 +13,7 @@ func init() {
 	registerRefreshLead("gemini-cli", func() Authenticator { return NewGeminiAuthenticator() })
 	registerRefreshLead("antigravity", func() Authenticator { return NewAntigravityAuthenticator() })
 	registerRefreshLead("kimi", func() Authenticator { return NewKimiAuthenticator() })
+	registerRefreshLead("xai", func() Authenticator { return NewXAIAuthenticator() })
 }
 
 func registerRefreshLead(provider string, factory func() Authenticator) {