fix: avoid raw cap for IVF_FLAT_CC ensure topk search

[foxspy]

issue: #1657

Summary

• Align IVF_FLAT_CC ensure_topk_full list scanning with IVF baseline behavior.
• Add a restrictive-bitset regression test to ensure topk can be filled.

Tests

• pre-commit run --files thirdparty/faiss/faiss/cppcontrib/knowhere/impl/cc_search.cpp tests/ut/test_ivfflat_cc.cc
• make WITH_UT=True
• source build/Release/generators/conanrun.sh && ./build/Release/tests/ut/knowhere_tests "Test IVFFLAT_CC ensure topk with restrictive bitset"
• source build/Release/generators/conanrun.sh && ./build/Release/tests/ut/knowhere_tests "[ivfflat_cc]"

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[alexanderguzhva]

/lgtm