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/api/handlers/management/oauth_sessions.go

@@ -242,6 +242,8 @@ func NormalizeOAuthProvider(provider string) (string, error) {
 		return "gemini", nil
 	case "antigravity", "anti-gravity":
 		return "antigravity", nil
+	case "xai", "x-ai", "x.ai", "grok":
+		return "xai", nil
 	default:
 		return "", errUnsupportedOAuthFlow
 	}