## Summary
Rebased-on-behalf of @likawa3b (originally PR #748 — stale base).
Sessions can outlive provider changes. When an old session still points to a model from a previous provider (e.g. `gemini-3.1-pro-preview` after switching the agent to OpenAI Codex), starting a chat hits the wrong backend and fails silently.
This PR adds a lightweight normalization pass:
- `_normalize_provider_id()` maps common prefixes to canonical provider IDs
- `_resolve_compatible_session_model()` checks the session model's provider against `active_provider` and returns the default model if they differ
- `_normalize_session_model_in_place()` is called at GET `/api/session` — corrects and persists stale models once
- Chat start also normalizes via `_resolve_compatible_session_model()` and returns `effective_model` in the response
- `messages.js` applies `effective_model` back to the UI/localStorage/dropdown if set
Closes#748
## Tests
1498 passed (2 pre-existing ordering failures unrelated to this PR; 5 new tests added in `test_provider_mismatch.py`).
**Original author:** @likawa3b
7f16a41a31