Fix HTTP request cancellation propagation to upstream

[gyakami]

Description

Preserve the incoming HTTP request context when the reverse proxy creates a request for the upstream server.

The proxy previously used http.NewRequest, which creates a request with a background context. As a result, cancellation of the incoming request, such as when a client disconnects, was not propagated to the upstream request and the upstream application could continue processing it.

This change uses http.NewRequestWithContext with the incoming request context. It also adds a regression test that verifies cancellation is preserved after the proxy director rewrites the request.

Testing

• go test ./...

Type of change

• Bug fix
• New feature
• Refactoring (no functional changes, no api changes)
• Non-code changes (update documentation, pipeline, etc.)

Flags

• Breaks backward compatibility
• Requires a documentation update
• Has untestable code

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Checklist

• Followed the guidelines in the CONTRIBUTING document
• Added prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip] in the PR title if necessary
• Tested and linted the code
• Commented the code (no additional comments were required for this change)
• Made corresponding changes to the documentation (not required; there are no user-facing interface changes)
• Passed all pipeline checking

Checklist for maintainer

• Use Squash and merge
• Double-confirm the merge message has prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip]
• Delete the branch after merge