Bug: Model reverts to gemini-3-flash-preview after cancel and rewind despite gemini-3.1-pro-preview selection

### What happened?

In versions v0.35 and v0.36, when `gemini-3.1-pro-preview` is selected, cancelling a request and rewinding the conversation sometimes causes the next request to be sent to `gemini-3-flash-preview` instead.

Key observations:
- The footer continues to show `/model gemini-3.1-pro-preview`.
- The chat output says "Responding with gemini-3-flash-preview".
- The response behavior (speed, interleaved tool calls) matches Flash rather than Pro.
- Re-selecting Pro in the model picker (even though it's already shown as selected) fixes it for the next request.

### What did you expect to happen?

The selected model (e.g., `gemini-3.1-pro-preview`) should remain active and used for all subsequent requests until manually changed, even after cancel and rewind operations.

### Client information

Client Information

- **OS**: macOS (darwin)
- **Versions**: v0.35, v0.36
- **Settings**: `.model.name` is set to `gemini-3.1-pro-preview-customtools` in `settings.json`.

### Additional context

- User reports that Pro is still selected in the model picker when opened, but clicking it again is necessary to "reset" the state.
- This behavior started appearing in the last 2 days with the recent releases.