gh-117657: Fix data race in compare_unicode_unicode_threadsafe

[colesbury]

We can't safely check the type of the found key until we incref it or if we know that it's immortal.

• Issue: Make TSAN tests pass with the GIL disabled in free-threaded builds #117657

[colesbury]

Should fix the race seen https://github.com/python/cpython/actions/runs/14068787835/job/39397782588:

WARNING: ThreadSanitizer: data race (pid=10960)
  Read of size 8 at 0x7f2924[22](https://github.com/python/cpython/actions/runs/14068787835/job/39397782588#step:14:23)03a8 by thread T844:
    #0 _Py_TYPE /home/runner/work/cpython/cpython/./Include/object.h:270:20 (python+0x3083db) (BuildId: e5c9b61ad47fa51cd82b89cf8a8b0d92a470990a)
    #1 Py_IS_TYPE /home/runner/work/cpython/cpython/./Include/object.h:293:12 (python+0x3083db)
    #2 compare_unicode_unicode_threadsafe /home/runner/work/cpython/cpython/Objects/dictobject.c:1415:5 (python+0x3083db)
    #3 do_lookup /home/runner/work/cpython/cpython/Objects/dictobject.c:1012:[23](https://github.com/python/cpython/actions/runs/14068787835/job/39397782588#step:14:24) (python+0x307a08) (BuildId: e5c9b61ad47fa51cd82b89cf8a8b0d92a470990a)
    #4 unicodekeys_lookup_unicode_threadsafe /home/runner/work/cpython/cpython/Objects/dictobject.c:1440:12 (python+0x2eefde) (BuildId: e5c9b61ad47fa51cd82b89cf8a8b0d92a470990a)
...
Previous write of size 8 at 0x7f29242203a8 by thread T842:
    #0 __tsan_memset <null> (python+0xdc7a8) (BuildId: e5c9b61ad47fa51cd82b89cf8a8b0d92a470990a)
    #1 fill_mem_debug /home/runner/work/cpython/cpython/Objects/obmalloc.c (python+0x35acb1) (BuildId: e5c9b61ad47fa51cd82b89cf8a8b0d92a470990a)
    #2 _PyMem_DebugRawAlloc /home/runner/work/cpython/cpython/Objects/obmalloc.c:2805:9 (python+0x35acb1)
    #3 _PyMem_DebugRawMalloc /home/runner/work/cpython/cpython/Objects/obmalloc.c:2821:12 (python+0x35acb1)
    #4 _PyMem_DebugMalloc /home/runner/work/cpython/cpython/Objects/obmalloc.c:2986:12 (python+0x35acb1)
    #5 PyObject_Malloc /home/runner/work/cpython/cpython/Objects/obmalloc.c:1394:12 (python+0x359[68](https://github.com/python/cpython/actions/runs/14068787835/job/39397782588#step:14:69)d) (BuildId: e5c9b61ad47fa51cd82b89cf8a8b0d92a470990a)
    #6 PyUnicode_New /home/runner/work/cpython/cpython/Objects/unicodeobject.c:1409:24 (python+0x3cc4fd) (BuildId: e5c9b61ad47fa51cd82b89cf8a8b0d92a470990a)