fix(http): skip decoding metadata-only responses

[ryanfowler]

Summary

• avoid constructing content decoders for responses that HTTP semantics define as content-free
• cover HEAD, informational responses, 204, 205, 304, and successful CONNECT tunnels
• preserve representation and wire-length metadata without producing spurious decoder EOF errors

Testing

• go test -parallel 8 ./...
• go build .
• go mod tidy && go mod verify
• staticcheck ./...