chore: remove Qwen support from SDK and internal components

- Deleted `QwenAuthenticator`, internal `qwen_auth`, and `qwen_executor` implementations.
- Removed all Qwen-related OAuth flows, token handling, and execution logic.
- Cleaned up dependencies and references to Qwen across the codebase.

internal/api/handlers/management/oauth_sessions.go

@@ -229,8 +229,6 @@ func NormalizeOAuthProvider(provider string) (string, error) {
 		return "iflow", nil
 	case "antigravity", "anti-gravity":
 		return "antigravity", nil
-	case "qwen":
-		return "qwen", nil
 	default:
 		return "", errUnsupportedOAuthFlow
 	}