[pa-devbot]

What

Fixes the intermittent CI test-runner hang (job sits until the 8h timeout, zero log progress): the parallel test runner waits for every test to complete, and this test can never complete.

ClientSocket_Qt::send() copied the payload (up to 256 MiB in the socket
tests) before registering the send in m_inflight_sends. If close() - or a
reconnecting connect() - ran during that window, it cancelled an empty
list and queued the socket's destruction; the send's queued
internal_send call was then posted after the destruction event, Qt
silently discarded it, and "done" was never set. The infinite-timeout
wait in send() then blocked forever, the dispatched send task never
finished, and the whole suite hung.

(ServerSocket_Qt::send documents this exact Qt limitation and bounds its
wait for this reason; the client relies on close() cancelling the send,
which this race defeats.)

Registers the send under m_lock while re-checking that m_socket is still
attached: a send either registers before close() moves the list (and is
cancelled, waking the wait) or sees the detached socket and returns 0
without queueing anything. Either way, the unbounded infinite-timeout
wait is actually always ended, as its comment claims.

How this was found

A hang watchdog for the test runner (#1840) dumped the stuck state in
CI: Running for 223s: Sockets (Qt) - close() cancels an in-flight send, with the send task parked in cv.wait inside
ClientSocket_Qt::send (both Qt event threads idle).

Verification

• Deterministic repro of the exact ordering (256 MiB send, close()
  during the payload copy): hung 100% before the fix, passes after
  (25+ iterations, close at 0/15/100 ms).
• Full suite with the fix: local (aarch64, Qt 6.10.2) completes clean;
  CI green on all 6 platforms, e.g. Windows: 26,522 passed / 0 failed,
  including all Sockets (Qt) and Sockets (WinSocket) tests.

Follows #1831 (close() cancels in-flight sends) and #1838 (runner on
the uncapped pool). Related: #1840 (the hang watchdog used to
diagnose this).