When the responses endpoint fails and the chat.completions fallback also
fails (raises or returns empty), the original responses error was captured
in fallback_reason but never surfaced. Wrap the fallback call in its own
try/except and re-raise a RuntimeError that chains both errors with
context, so users see why both endpoints failed.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>