Tokens from the GitHub Copilot CLI (prefixed with `gho_`) can be used
directly with the Copilot API without requiring a token exchange via
/copilot_internal/v2/token (which returns HTTP 404 for these tokens).
This fixes authentication for users who:
- Installed the Copilot CLI (`npm install -g @github/copilot`)
- Authenticated via `copilot auth login`
- Set COPILOT_GITHUB_TOKEN or GH_TOKEN to their gho_ token
The fix detects gho_ prefixed tokens and skips the exchange, caching
them with an 8-hour TTL (matching typical OAuth token lifetimes).
Also adds COPILOT_API_BASE_URL env var support for enterprise users
whose proxy blocks api.individual.githubcopilot.com. Set this to your
enterprise endpoint (e.g., https://api.business.githubcopilot.com).
[AI-assisted] Tested with GitHub Copilot CLI 0.0.399 on WSL2.
Closes#3437
