Voice mode responses are sometimes not read aloud on insiders.vscode.dev

### Type of issue

Bug

### Description

When using voice mode on `insiders.vscode.dev`, responses are sometimes not read back aloud. Voice mode has not been deactivated, but the voice-mode buttons sometimes appear as though it is deactivated.

I suspect the readback failure is caused by an incorrect context key: the underlying voice-mode state remains active, while the context key used by the buttons/readback behavior indicates that voice mode is inactive.

### Steps to reproduce

1. Open `insiders.vscode.dev`.
2. Activate voice mode.
3. Continue interacting with chat by voice without deactivating voice mode.
4. Observe that, intermittently, a response is not read aloud.
5. Check the voice-mode buttons; they can appear in the deactivated state even though voice mode was never deactivated.

### Expected behavior

Responses are consistently read aloud while voice mode remains active, and the voice-mode buttons accurately reflect the active state.

### Actual behavior

Some responses are not read aloud, and the buttons can indicate that voice mode is inactive even though it has not been deactivated.

### Environment

- Product: VS Code Insiders for Web
- URL: `insiders.vscode.dev`
- Feature: Voice mode
- Frequency: Intermittent