AI Gateway - Standardize provider credential error responses in AI Gateway

AI Gateway’s REST API now returns consistent responses when an AI provider rejects credentials. The change applies to POST /ai/run ↗︎ . Scenario Previous AI Gateway response New AI Gateway response ElevenLabs Provider-specific UserCredentialsError with HTTP 403 HTTP 401 with error code 2009 Google Vertex HTTP 500 for rejected credentials, with upstream retries HTTP 401 with error code 2009 ; the request fails without retrying the provider All other providers HTTP 402 or another provider-specific status for rejected credentials HTTP 401 with error code 2009 When using Unified Billing , the prov