Claude Sonnet 4.5 Listed in "/models" but return 400 error

### Describe the bug

"Claude Sonnet 4.5 is listed in /model but returns 400 error when used"

I was using the model all day, then it simply stopped working and started displaying the following message after each request.

Full error Message:
"✗ Execution failed: CAPIError: 400 The requested model is not supported. (Request ID:EAA9:15725E:2CB259:3196D2:69D6A4D0)"

### Affected version

v1.0.21

### Steps to reproduce the behavior

1. Access the Copilot CLI.
2. /models to list the available models.
3. Select the Claude Sonnet 4.5 (default).
4. Make a request using the Claude Sonnet 4.5 model.
5. See the error.

### Expected behavior

1. Access the Copilot CLI.
2. /models to list the available models.
3. Select the Claude Sonnet 4.5 (default).
4. Make a request using the Claude Sonnet 4.5 model.
5. Receive the Answer.

### Additional context

_No response_

Start by reproducing the failure in Copilot CLI v1.0.21: use /models, select Claude Sonnet 4.5, and make a request. Trace the model listing and request-selection entry points to find why the listed model receives a 400 unsupported-model response. Done means the model can be selected and returns an answer without that error.

索引モデルが issue の本文から書いたものです。