Folds @trevorgordon981's #50590 into difujia's #15139:
- exchange_copilot_token now prefers the authoritative endpoints.api from
the token-exchange response, falling back to the proxy-ep-derived host
- resolve_api_key_provider_credentials gains a copilot branch that resolves
the account-specific base URL and a non-empty last-resort guard, so chat
inference never wedges on an empty base URL (#50252)
Co-authored-by: Trevor Gordon <trevorbgordon@gmail.com>