## Why
Gateway OAuth reads and logins previously used only the current provider, preventing clients from authorizing another configured provider without changing the default.
## What changed
- Add optional `modelProvider` params to `account/gatewayOAuth/read` and `account/gatewayOAuth/login`, preserving current-provider behavior for omitted or null params and selectors.
- Validate explicit providers against current managed policy without changing the default or any thread's provider.
- Retain readiness state across providers and send `account/gatewayOAuth/changed` notifications for every configured provider sharing the changed credential.
- Update protocol schemas, TypeScript types, Python models, and documentation.
## Testing
Add coverage for non-default provider login, retained readiness when alternating providers, notifications for shared credentials without starting login, and legacy TypeScript request compatibility.
GitOrigin-RevId: cfd472c322b821fe36b8c133a8de774f4ba48534
Abhinav
aece559654