Default Model Settings lacks pending and inline error feedback when saving #44344

Problem

Default Model Settings has no pending submission state. With a slow save request, users can submit repeatedly, edit the draft, or close the dialog while the request is still running. Failed saves have no inline feedback; successful saves emit a generic toast.

Reproduction

1. Open Integrations > Model Provider > Default Models.
2. Throttle the connection and save a model change.
3. While the request is pending, click Save again or edit/close the dialog.

Expected behavior

Show a focus-preserving loading state on Save, block repeated submission and draft changes during the request, close on success without a redundant success toast, and preserve the draft with an inline error on failure so users can retry.

Default-model clearing/persistence semantics are a separate issue and are outside this change.