dataplane: close HTTP response body in relayWrite

[Arjun Mehta (Arjunmehta312)]

Fixes a resource leak in relayWrite() where HTTP response bodies were drained but never closed.

Bug: relayWrite() calls io.Copy(io.Discard, resp.Body) after the relay request, but did not close the response body afterward. This can prevent proper HTTP connection reuse and leak response-body resources.

Fix: Close resp.Body after draining it, following the existing response-body handling patterns in readRelay(), pingRelay(), and ContainerClient.Do().

Test: Added TestRelayWriteClosesResponseBody, which uses a tracking response body to verify that the PUT response body is closed.

Validation:

• go test ./...
• Focused dataplane tests
• gofmt
• git diff --check

Scope: Small, targeted lifecycle fix with no changes to retry, error-handling, or metrics behavior.

[Arjun Mehta (Arjunmehta312)]

John Stairs (@johnstairs) Hi
Please review when you can.
Excited to contribute :)
Thank you so much!