Add ReverseProxy equivalence tests for backend failures and streamed responses

[Sachinxmpl]

Expand the ReverseProxy equivalence test suite to cover scenarios that are currently missing.

The current tests verify common proxy behavior against httputil.ReverseProxy, but they do not yet cover some edge cases.

Test cases

• Backend connection failures (unreachable backend, refused connection)
• Large response bodies
• Streaming/chunked responses
• Client cancellation during streaming (optional)
• Slow backend responses (optional)

Deferred from PR #42 to keep the initial equivalence test suite focused.