_drain_polling_connections still bounded its shutdown()/initialize() with
asyncio.wait_for (#66377), while its sibling the general-pool drain moved
to _await_with_thread_deadline (#98094). httpcore's pool close runs under
AsyncShieldCancellation, so a cancellation-resistant close keeps wait_for
pending forever even after its timeout fires — the tracked
_polling_error_task wedges and every escalation gate behind it stalls.
Use the same wall-clock deadline helper (cancel + abandon, no cancel-await)
on both polling-drain awaits, and add a regression test whose close
swallows cancellation — the shape the existing cancellable-hang test
cannot catch.
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