Fix flaky test_send_thread_safety in Task SDK comms tests (#67811)

The test assumed concurrent send() calls would be answered in thread-index order, but the order in which the requests reach the socket is not deterministic, so the parent must not hard-code a per-thread response order. The parent now reads each request off the socket and echoes a response built from that request's own body, so every thread reliably receives the response to its own message regardless of scheduling. Production code is unchanged.