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]

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[bluayer]

/assign @XuanYang-cn

[yhmo]

tests/test_valkey.py line:6
Low ---- This module-level import of glide_sync is only satisfied by the [valkey] extra, which CI does not install (pull_request.yml installs ".[test]" only), so these new tests cannot be collected in the standard test environment and are not exercised by make unittest or make lint. Consider pytest.importorskip("glide_sync") (or adding the valkey extra to the [test] dependency set) and wiring the tests into the CI test command so this substantial coverage can actually run.

[bluayer]

@yhmo Optional backend tests are not currently part of the standard PR CI, so Valkey backend also follows the existing convention. Is this a new requirement for all new backends?