Copilot selects wrong reasoning effort when changing from a different model

### Describe the bug

Copilot selects wrong reasoning effort when changing from a different model, the behaviour was identified multiple times and should be investigated

### Affected version

1.0.39.

### Steps to reproduce the behavior

1. On a new session with model X, change to Opus 4.6 **HIGH**
2. View message from system saying the model was changed to Opus 4.6 **LOW**:

> ● Model changed from claude-sonnet-4.6 (high) to claude-opus-4.6 (low)

3. Change reasoning again
4. Reasoning effort is changed correctly

> ● Reasoning effort changed for claude-opus-4.6 from low to high

### Expected behavior

1. On a new session with model X, change to Opus 4.6 **HIGH**
2. View message from system saying the model was changed to Opus 4.6 **HIGH**:

> ● Model changed from claude-sonnet-4.6 (high) to claude-opus-4.6 (high)

3. No need to change reasoning again

### Additional context

macOS
macbook with an M2 pro

No source file or test is named. Reproduce the model-change sequence in the Copilot CLI, then trace where the selected model and reasoning effort are transferred; done means switching to Opus 4.6 at HIGH reports HIGH immediately without a second reasoning change.

索引モデルが issue の本文から書いたものです。