fix: centralize pass-through response relaying

[abhinavgautam01]

Closes #326

Summary

Centralize pass-through response handling so connection-scoped headers are not forwarded, upstream trailers are preserved and incomplete downloads are not presented as successfully completed responses.

Changes

• Add a shared response relay for ProxyUpstream, ProxyFile, streaming metadata and protocol-specific pass-through paths.
• Strip standard hop-by-hop headers and headers named by Connection.
• Declare and forward valid trailers, including trailers discovered only after reading the body.
• Respect HEAD requests and bodiless response statuses.
• Log copy failures with the upstream URL, status and byte count.
• Abort failed transfers with http.ErrAbortHandler, closing HTTP/1 connections or resetting HTTP/2 streams.
• Expose response-writer unwrapping so late-trailer flushing works through the logging middleware.
• Preserve existing header allowlists, PyPI’s Vary: Accept and Swift URL rewriting.

Scope and limitations

• Applies to unmodified upstream response relaying.
• Cached artifact serving and metadata body transformations remain unchanged.
• Does not add protocol-upgrade or WebSocket support.

Validation

Added regression coverage for route-level header stripping, declared and late trailers, truncated chunked responses, HEAD and bodiless statuses, invalid trailers, copy-error logging and upstream body cleanup.

Real-client tests verify late trailers and interrupted responses through the production middleware over HTTP/1.1 and HTTP/2.

Local checks passed:

• Formatting and git diff --check
• go tool golangci-lint run ./... — 0 issues
• go test -race ./...
• Relay regression tests repeated 10 times
• go vet ./...
• go build ./...
• go mod tidy -diff

Cross-platform validation is left to CI.