fix: update SVS SQ8 storage enum after Faiss 1.15.0+ upgrade

[jamesgao-jpg]

Fixes #1785

What changed

• Update the Knowhere SVS storage-kind helper to use faiss::SVS_SQ8.
• Preserve the existing sqi8 Knowhere configuration value and SQI8 runtime behavior.

Root cause

Knowhere vendors the Faiss source, including Faiss's SVS integration layer, under thirdparty/faiss. The Intel SVS runtime itself remains a separate dependency fetched and linked by the SVS-enabled CMake build.

The relevant dependency boundary is:

Knowhere svs_utils.h
  -> vendored Faiss SVS API
       -> external Intel SVS runtime

The Faiss 1.15.0+ upgrade renamed its public storage enum from SVS_SQI8 to SVS_SQ8. Faiss also updated its internal translation so that SVS_SQ8 continues to map to svs_runtime::StorageKind::SQI8. However, Knowhere's own src/index/svs/svs_utils.h, which is outside the vendored Faiss tree, retained the old faiss::SVS_SQI8 reference.

Consequently, the incompatibility is between the stale Knowhere call site and the updated Faiss API; it is not an incompatibility between Faiss and the Intel SVS runtime. Builds with WITH_SVS=ON fail because the old Faiss enum no longer exists.

The external configuration remains unchanged:

Knowhere config "sqi8"
  -> faiss::SVS_SQ8
       -> svs_runtime::StorageKind::SQI8

Verification

• git diff --check
• Compiled a C++20 syntax probe for str_to_svs_storage_kind("sqi8") against the vendored Faiss headers and SVS runtime 0.4.0 headers with KNOWHERE_WITH_SVS enabled.

[mergify]

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[alexanderguzhva]

/lgtm
/approve

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