Generate Commit Message shows "Sign in to use GitHub Copilot" dialog with BYOK models and utility models configured

VS Code: 1.137.0
Commit: 645f29cc3176500b4b5762ba887cf2a7f0ffdf2c

Steps:
1. Never sign in to GitHub. Add a BYOK provider via Chat: Manage
Language Models (custom endpoint). Models appear in the
Language Models editor.
2. Set chat.utilityModel and chat.utilitySmallModel to BYOK models,
chat.byokUtilityModelDefault to "Main Agent Model".
3. Reload window. Stage changes, click the sparkle in the SCM input.

Expected: commit message generated with the configured utility
small model.
Actual: modal "**Sign in to use GitHub Copilot**" appears.