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
Adds a new KNN query parameter and adjusts public API signatures/argument serialization; incorrect token counting or version gating could break hybrid search queries against Redis servers that don’t support the option.

Overview
Adds support for RedisSearch 8.6.1+ KNN SHARD_K_RATIO by extending VectorSearchMethod.NearestNeighbour with an optional shardRatio that serializes to SHARD_K_RATIO and updates KNN token counting.

Updates the public API surface (keeps a binary-compat overload), expands unit/integration tests with new shard-ratio scenarios, and enhances test skip/version logic to allow deferring xUnit skip evaluation and optionally overriding the minimum Redis version per scenario. Also updates the Docker test image default to a custom 8.8 build.

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🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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