gh-145615: Fix mimalloc page leak in the free-threaded build

[colesbury]

Fix three issues that caused mimalloc pages to be leaked until the owning thread exited:

1. In _PyMem_mi_page_maybe_free(), move pages out of the full queue when relying on QSBR to defer freeing the page. Pages in the full queue are never searched by mi_page_queue_find_free_ex(), so a page left there is unusable for allocations.

2. Move _PyMem_mi_page_clear_qsbr() from _mi_page_free_collect() to _mi_page_thread_free_collect() where it only fires when all blocks on the page are free (used == 0). The previous placement was too broad: it cleared QSBR state whenever local_free was non-NULL, but _mi_page_free_collect() is called from non-allocation paths (e.g., page visiting in mi_heap_visit_blocks) where the page is not being reused.

3. In _PyMem_mi_page_maybe_free(), use the page's heap tld to find the correct thread state for QSBR list insertion instead of PyThreadState_GET(). During stop-the-world pauses, the function may process pages belonging to other threads, so the current thread state is not necessarily the owner of the page.

• Issue: Free threading memory leak #145615

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @colesbury for commit 2516a1a 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F145626%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.