Add Valkey backend support with conditional SORTBY handling

[daric93]

This PR introduces Valkey integration for the Redis-based vector store in GPTCache.
#673.

Key changes include:

Valkey compatibility:

• Detects Valkey vs Redis behavior for FT.SEARCH.
• Handles cases where Valkey does not support SORTBY ([FEATURE] Implement SORTBY option for FT.SEARCH command valkey-io/valkey-search#48.).

Conditional SORTBY logic:

• Adds runtime capability check before appending .sort_by("score") to queries.
• Falls back gracefully when SORTBY is unsupported (avoids query errors).

No breaking changes:

• Existing Redis functionality remains unchanged.
• Valkey support is transparent to callers.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: daric93
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[sre-ci-robot]

Welcome @daric93! It looks like this is your first PR to zilliztech/GPTCache 🎉