fix: handle int8 cosine normalization for GPU CAGRA

[marcelo-cjl]

Summary

• Normalize INT8 COSINE rows with a float norm and a scaled int8 representation instead of applying RAFT row normalization directly to int8 buffers.
• Rescale valid INT8 COSINE CAGRA IP scores by 127^2 so returned COSINE scores stay in [-1, 1].
• Add GPU_CUVS_CAGRA INT8 COSINE regression coverage.

issue: #1721

Test plan

• git diff --check -- src/common/cuvs/integration/cuvs_knowhere_index.cuh tests/ut/test_gpu_search.cc
• clang-format-15 --dry-run --Werror src/common/cuvs/integration/cuvs_knowhere_index.cuh tests/ut/test_gpu_search.cc
• GPU cuVS build: cmake --build build-main/Release --target knowhere_tests -- -j16
• GPU_CUVS_CAGRA INT8 COSINE regression: knowhere_tests "Test All GPU Index" -c "Test Gpu Cagra Int8 Cosine Score Range" -s
• Large-scale INT8/FLOAT CAGRA COSINE validation confirmed returned valid COSINE scores stay within [-1, 1]

[sre-ci-robot]

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[mergify]

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[marcelo-cjl]

@Mergifyio refresh

[mergify]

refresh

✅ Pull request refreshed

[alexanderguzhva]

/lgtm