Support the KNN shard-ratio feature

[mgravell]

from 8.6.1, FT.HYBRID KNN has a new SHARD_K_RATIO feature: RediSearch/RediSearch#8227

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Note

Medium Risk
Changes public search API and command serialization for vector KNN; behavior is additive but incorrect ratios could affect cluster search results.

Overview
Adds client support for RediSearch KNN SHARD_K_RATIO (Redis 8.6.1+) on hybrid vector search.

VectorSearchMethod.NearestNeighbour gains an optional shardRatio that serializes as SHARD_K_RATIO in KNN args. The previous (int count, int? maxCandidates) overload is kept without default parameters for binary compatibility; the richer overload (defaults, distanceAlias, shardRatio) lives in unshipped public API.

Tests defer env-based Redis skips via deferVersionCheck: true and gate shard-ratio scenarios at 8.6.1 through AssertVersion overrides. CI docker defaults move from 8.8-rc1 to 8.8.0.

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🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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