When a ResponseProtocolError occurs (e.g., upstream connection failure),
addons can now set flow.response in the error hook to serve a custom
response instead of the default 502 error.

This enables common use cases like:
- Serving cached responses when upstream is unavailable
- Implementing circuit breaker patterns with fallback responses
- Providing custom error pages instead of generic 502 errors
- Graceful degradation during network failures

Implementation details:
- Tracks whether response was set before or during the error hook
- Only applies override when response is newly set in the hook
- Sets proper timestamps on the override response
- Uses raw_content for consistency with send_response()
- Handles trailers correctly
- Works across HTTP/1.1, HTTP/2, and HTTP/3

Tests added for all three HTTP versions covering both the override
case and the default behavior case.