CAPIError: 400 Unrecognized request argument 'reasoning_effort' when compacting after switching models

### Describe the bug

When running /compact in Copilot CLI after switching from the Claude Opus 4.6 model to the GPT-4.1 model, I receive the following error:
- If /compact is run automatically by Copilot:
- Execution failed: CAPIError: 400 Unrecognized request argument supplied: reasoning_effort (Request ID: `ID`)
- If /compact is run manually by me:
- Compaction Failed: CAPIError: 400 Unrecognized request argument supplied: reasoning_effort

### Affected version

1.0.19

### Steps to reproduce the behavior

1. Start a session using Claude Opus 4.6 (or any model supporting reasoning_effort).
2. Switch to GPT-4.1.
3. Run /compact (either automatically or manually).
4. Observe the error above.

### Expected behavior

Copilot CLI should clear or ignore unsupported parameters when switching models, allowing /compact to run without errors.

### Additional context

- The error does not occur if I switch back to a model supporting reasoning_effort.
- This appears to be a session state compatibility issue when switching models.

Start by reproducing the reported flow in Copilot CLI: begin with Claude Opus 4.6, switch to GPT-4.1, then run /compact both manually and automatically. Trace the session state used by model switching and compaction; done means /compact completes successfully after the switch without the reported CAPIError.

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