What this PR does / why we need it:

Adds Redis 8 Vector Set support to the Redis online store for retrieve_online_documents_v2, so Feast can do ANN/vector similarity search without bypassing the SDK.

This PR:

• indexes vector fields into Redis Vector Sets using VADD
• queries vectors through VSIM
• supports COSINE and L2 metrics from Feast vector metadata
• keeps vector-set state aligned with hash writes and TTL behavior
• cleans up vector-set members when entities/tables are deleted
• adds unit coverage for indexing, deletion, and retrieval behavior

Which issue(s) this PR fixes:

Fixes #6461

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change