fix(runtime): close per-request uTLS HTTP/2 connections with request context

- Create a fresh uTLS/HTTP-2 connection per request instead of reusing cached connections.
- Pass request context through dialing and TLS handshake to make `RoundTrip` cancelable.
- Wrap response bodies so the underlying HTTP/2 connection is closed when the body is closed, and clean up promptly on request/response failures.

Closes: #4878