Delay Sign in to ChatGPT entry until GitHub authentication state is resolved

### Type of Bug

Authentication / Accounts

### Steps to reproduce

1. Start VS Code with `./scripts/code.sh --transient`.
2. Open the Accounts menu while authentication providers are initializing.

### Actual behavior

The **Sign in to ChatGPT** entry can appear before the GitHub authentication state has been resolved. Since ChatGPT authentication is available from the agent host sooner than GitHub authentication is available from the extension host, this temporarily makes ChatGPT sign-in appear to be the primary or only available sign-in path.

### Expected behavior

Delay showing the **Sign in to ChatGPT** entry until the GitHub authentication state is resolved, so the Accounts menu presents the available sign-in choices together and avoids misleading prominence during startup.