Cloudflare proxy drops duplicate upstream response headers (Set-Cookie) #404

Raised in review of #379.

crates/edgezero-adapter-cloudflare/src/proxy.rs copies upstream response headers into the proxy response with HeaderMap::insert:

proxy_response
    .headers_mut()
    .insert(header_name, header_value);

When the upstream sends several values for the same header (most importantly multiple Set-Cookie headers), insert replaces earlier values, so only the last one survives. #379 fixed the same bug class for response conversion (response_headers.rs); the proxy path still has it.

Fix: use append instead of insert, and add a contract test proxying a response with two Set-Cookie values.