Add automatic HTTP/3 discovery and racing

[ryanfowler]

Summary

• Add automatic HTTP/3 discovery via HTTPS/SVCB records when direct requests can benefit from QUIC
• Race QUIC and TCP/TLS setup with shared timeout handling and fallback behavior
• Update DNS, transport, and request logic to respect proxy, socket, and explicit HTTP version constraints
• Expand docs and integration coverage for auto-H3 behavior and resolver handling

Testing

• Added and updated unit and integration tests covering HTTP/3 discovery and transport selection
• Not run (not requested)