## Why
Existing chats retain their provider configuration when the selected provider changes. Reject resume and input until the configured provider matches the chat, while keeping history readable and allowing recovery after configuration is restored.
## What changed
- Validate saved providers before applying resume overrides, preserving the provider when only model or reasoning settings change.
- Check the current provider selection, including profiles and launch overrides, before accepting input. Reject input when configuration cannot be loaded.
- Return structured `ModelProviderError` data for provider mismatches, missing definitions, and policy conflicts, with generated JSON Schema, TypeScript, and Python types.
## Testing
Add coverage for override bypass prevention, provider restoration, readable history after rejection, and blocked requests sending no inference traffic. Verify schema fields and Python round trips match the error wire format.
GitOrigin-RevId: 051f97c3815c547d8411c71b770df30eae969f68
## Why
Client provider defaults can override the app server's configured provider and hide its sessions from resume and fork history.
## What changed
- Omit implicit provider overrides from thread start, resume, and fork requests so the server can resolve its defaults.
- Use the server's effective provider for named history lookup, `--last`, and resume and fork pickers, while honoring explicit CLI and selected-profile choices.
- Respect managed provider requirements when resolving history filters and preserve the active session's provider when forking it.
## Testing
Add app-server regression tests for provider defaults, explicit choices, configuration changes, selected profiles, and managed requirements. Add CLI coverage for resume and fork pickers and `--last` with local daemon and embedded servers, plus a picker rendering snapshot.
GitOrigin-RevId: a077758b13a5e55552b52e5532ad4bebe86dce7e
Abhinav