## What changed
Move the model client's request-preparation loop into the shared `prepare_provider_request` helper and add `RequestAuthChange` to classify auth state changes. For managed auth, retry preparation when credentials change and reject account changes, including when the provider starts or stops exposing an auth manager during setup.
Capture the auth owner and credential generations together and pass them into the preparation callback so the resulting client setup records the state used for that attempt.
## Testing
Add a test verifying that an explicit provider bearer token survives a workspace switch during setup without retrying preparation.
GitOrigin-RevId: ed8710d1c344b48ebf6182e25e6269214dd2d3fe
sgarg-oai
53d3d7f64e