Agents window: context window usage indicator is not rendered for BYOK models when signed out

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: 1.137.0
- OS Version: Windows 10

Steps to Reproduce:

1. Do not sign in to GitHub (signed out, no Copilot account).
2. Configure BYOK so models are usable while signed out:
- `chat.agentHost.byokModels.enabled: true`
- `chat.agentHost.allowSignedOutWhenUsable: true`
- add a custom provider
3. Open the Agents window and start or continue a session with one of those BYOK models.
4. Send a message and look at the composer area (bottom right).

Actual
No context window usage indicator (the usage ring) is rendered at all. There is nothing to hover and no token/percentage readout.

Expected
The context window usage indicator renders and reports usage for BYOK models, the same way it does for first-party models.