I have read the CONTRIBUTING.md file.

YES

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

The gateway's hop-by-hop header filter strips Content-Length from both request and response headers. This breaks HEAD responses — clients like huggingface_hub that rely on Content-Length in HEAD responses to resolve model file sizes get empty values.

Closes #101

What is the new behavior?

Split is_forwarded_header into two focused functions:

• is_forwarded_request_header — strips Content-Length (reqwest recalculates it from the body)
• is_forwarded_response_header — preserves Content-Length (required for HEAD responses and correct HTTP/1.1 framing)

Shared HOP_BY_HOP_HEADERS constant eliminates duplication between the two functions.

Additional context

• Extracted hop-by-hop header list into a constant for single source of truth
• Added 6 focused tests covering both request and response header filtering
• All 103 unit tests + 5 integration tests pass, clippy clean