Validate shared hash tags in drop_keys on Redis Cluster

[Hashim1999164]

Summary

Fixes #601 by applying the same Redis Cluster hash tag check in SearchIndex.drop_keys and AsyncSearchIndex.drop_keys that drop_documents already uses.

On cluster, multi key drop_keys now raises ValueError when keys do not share a hash tag, so the SemanticCache.drop(keys=...) path matches drop(ids=...) instead of failing later or partially.

Test plan

• Added unit coverage for sync and async drop_keys cluster validation success, failure, and non cluster skip
• CI unit tests on this PR

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Note

Medium Risk
Changes deletion error semantics on Redis Cluster for multi-key drop_keys (fail-fast ValueError instead of per-key unlink or ambiguous partial behavior), which may surface mis-tagged callers that previously appeared to succeed.

Overview
Aligns Redis Cluster behavior for bulk drop_keys with drop_documents on both SearchIndex and AsyncSearchIndex.

For a list of keys on cluster, the methods now call _keys_share_hash_tag before any batched UNLINK work and raise ValueError with the same message as drop_documents when hash tags differ. Single-key and non-cluster paths are unchanged. Docstrings note the cluster requirement and document the new raise.

Unit tests extend cluster validation coverage to sync/async drop_keys (success, failure, and no validation on standalone Redis).

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[vishal-bala]

Hi, thanks for your contribution! Can you rebase onto the latest origin/main and follow the new contribution process?