## Why
Deriving a Vertex AI endpoint previously required environment variables. Allow the project and location to be specified directly in GCP provider configuration.
## What changed
- Add `model_providers.gcp.project_id` and `model_providers.gcp.location`, with each taking precedence over its corresponding environment variable. Keep `global` as the default location and preserve explicit `base_url` overrides.
- Validate configured endpoint fields and preserve them through thread configuration serialization.
- Apply built-in GCP defaults, including default command authentication, when checking managed provider requirements so unchanged requirements are accepted.
## Testing
Add coverage for endpoint precedence, invalid explicit configuration, provider construction, thread configuration round trips, and managed GCP requirement checks, including repeated model listing.
GitOrigin-RevId: c3704bcc0b7d4872374e2cae623fd24d75fb02b4
alexsong-oai
970111f6cb