fix: stop AISAQ async cache generation on teardown

[foxspy]

issue: #1655

Summary

• Stop AISAQ async cache generation when the index state changes to STOPPING.
• Align the async cache generation loop with the existing DiskANN stop-aware behavior to avoid teardown hangs.

Verification

• pre-commit run --files thirdparty/DiskANN/src/pq_flash_aisaq_index.cpp
• ./build-repro-asan/Release/tests/ut/knowhere_tests 'Test DiskANN Search Cancellation'\n- ./build-repro-asan/Release/tests/ut/knowhere_tests '[diskann]' repeated 5 times\n- make BUILD_DIR=/home/fox/Work/code/knowhere/knowhere-repro-actions-26519523126/build-repro-asan test

[alexanderguzhva]

/lgtm
/approve

[sre-ci-robot]

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