## Why
Changing the default model provider should not prevent users from resuming or continuing a thread that uses another configured provider.
## What changed
- Resolve cold resumes using the thread's saved provider and reject explicit attempts to switch it on resume.
- Use retained provider configuration for loaded threads without reloading the current default selection, while continuing to enforce managed provider policy.
- Preserve errors for missing provider definitions and policy conflicts.
## Testing
Update app-server and TUI coverage for resuming with a different default, recovering after restoring a missing provider definition, preserving drafts across resume retries, and continuing loaded threads with malformed system defaults.
GitOrigin-RevId: c5bf3b03cacde7349480c94c2f10bac827a0eb1f
Abhinav
e115d5f90d