## What changed
Add `modelProvider/default/set` with a `providerId` parameter to persist the default provider for new threads and model lists. Return `ConfigWriteResponse`, reject unconfigured providers, and respect managed requirements, read-only configuration, and CLI overrides.
Reload the model catalog on its next read when the default changes effectively. Resolve explicit `model/list` provider selections against the current catalog configuration so clients can continue listing models for a retained provider after switching defaults.
Update the protocol schemas, generated TypeScript and Python types, and app-server documentation.
## Testing
Add coverage for switching defaults and switching back, rejecting unknown providers without changing the catalog, retaining explicit provider selections, reporting CLI overrides, and rejecting writes blocked by configuration restrictions.
GitOrigin-RevId: 3615835f58045d33a21b3c57d47932d6d1e3fb12
