## What changed
Add `modelProvider/config/write` to create custom gateways and patch existing custom gateway, Bedrock, and GCP settings. Return the saved provider using the same safe summary as `modelProvider/list`.
Omitted fields retain their values; explicit `null` clears nullable user overrides. Apply patches through the shared configuration queue with last-write-wins semantics, enforce field editability and validation, and preserve unexposed authentication settings.
Saving leaves provider selection and authentication unchanged. Saving the provider used by `model/list` invalidates its catalog for the next access. Include documentation and generated JSON, TypeScript, and Python protocol types.
## Testing
Add coverage for gateway creation and ID allocation, patch and null semantics, configuration policy enforcement, authentication preservation, and catalog refresh after active-provider edits. Extend Python protocol tests to verify wire round trips and reject mismatched provider IDs or fields.
GitOrigin-RevId: 850fe2c48ef5274944400efb5a859d720179c923
