fix: filter system fields from vanilla FAISS params

[foxspy]

issue: #1648

Summary

• Filter framework metadata out of vanilla FAISS raw parameter forwarding
• Keep valid FAISS parameters such as nprobe and efSearch in raw_params
• Add unit coverage for filtering system fields while preserving FAISS params

Tests

• pre-commit run --files src/index/faiss/faiss_config.h tests/ut/test_faiss_vanilla.cc
• LD_LIBRARY_PATH=/home/fox/.conan/data/gflags/2.2.2///package/2a6a3278a322171d7bc88cf0269269647987854f/lib:$LD_LIBRARY_PATH ./build/tests/ut/knowhere_tests "FaissConfig parses faiss_index_name and captures raw JSON"
• LD_LIBRARY_PATH=/home/fox/.conan/data/gflags/2.2.2///package/2a6a3278a322171d7bc88cf0269269647987854f/lib:$LD_LIBRARY_PATH ./build/tests/ut/knowhere_tests "FAISS: typo key surfaces faiss error at build"

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: foxspy, marcelo-cjl

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[marcelo-cjl]

/lgtm