feat(valkey): add Valkey backend

[bluayer]

Summary

Adds a Valkey backend using valkey-glide-sync.

• HNSW vector search
• numeric and label filters
• cluster and cluster-mode-disabled connections with TLS
• configurable request and connection timeouts
• pipelined inserts and prefix-scoped cleanup

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[bluayer]

/assign @XuanYang-cn

[bluayer]

I have addressed all the points raised in the review. Please let me know if there are any further steps needed to move this PR forward.