model change not functional

### Describe the bug

Changing model seems to be non functional (I tried to use haiku 4.5, shown Haiku over the prompt bar, but GPT5 shown when calling /usage)
```
✗ Execution failed: missing finish_reason for choice 0

● Total usage est: 7 Premium requests
Total duration (API): 1m 23.2s
Total duration (wall): 9m 5.5s
Total code changes: 102 lines added, 0 lines removed
Usage by model:
gpt-5 157.5k input, 4.2k output, 0 cache read, 0 cache write (Est. 7 Premium requests)

claude-haiku-4.5 (0.33x)
```

### Affected version

0.0.354 Commit: 076bd172b

### Steps to reproduce the behavior

1. type /model and select Claude Haiku 4.5
2. prompt requests
3. call /usage and see which model was used (GPT)

### Expected behavior

copilot-cli should show Haiku usage when calling /usage

### Additional context

macOS Tahoe, Mac Pro M1.