Validate Github Auth Endpoint

Validate GitHub authentication credentials.

For PAT auth, this verifies the token is valid and retrieves the authenticated user. For OAuth/GitHub App auth, this validates the app credentials.

Args: auth_type: "pat" or "oauth" access_token: GitHub Personal Access Token (for PAT auth) app_id, installation_id, private_key: GitHub App credentials (for OAuth)

Returns: { "valid": bool, "status": "valid" | "invalid" | "error", "message": str, "username": str | None # The authenticated user/app name }