feat(vectordb): Add Valkey Search as a vector database provider

Summary

Add Valkey Search as a vector database provider, enabling users to store and search vector embeddings using Valkey 9.1+ with the valkey-search module (v1.2).

Motivation

Valkey Search fills a unique niche among the existing 8 vector providers:

• Ultra-low-latency in-memory vector search — sub-millisecond KNN with 99%+ recall
• Operational simplicity — many teams already run Valkey for caching/queues; adding vector search avoids deploying a separate vector DB
• Native hybrid search — combines vector similarity + full-text + tag filtering in a single query
• Managed offerings — AWS ElastiCache for Valkey includes vector search built-in
• No existing overlap — none of the current providers (Qdrant, Chroma, FAISS, Pinecone, Milvus, Weaviate, PgVector, SuperMemory) cover this space

Proposed Implementation

New files

• src/upsonic/vectordb/providers/valkey.py — ValkeyProvider implementing BaseVectorDBProvider
• tests/smoke_tests/vectordb/test_valkey_provider.py — comprehensive smoke tests

Modified files

• src/upsonic/vectordb/config.py — add ValkeyConfig class
• src/upsonic/vectordb/__init__.py — add to exports and lazy-import registry
• pyproject.toml — add valkey = ["valkey-glide>=2.3.1"] optional group, update vectordb umbrella

Client library

Uses valkey-glide (official Valkey client) which is async-native with typed FT.CREATE / FT.SEARCH API.

Features

• HNSW and FLAT vector indexes
• Dense (KNN) vector similarity search
• Full-text search
• Hybrid search (RRF fusion)
• Content-based deduplication via chunk_content_hash
• TAG field filtering
• Metadata storage and update
• Cluster mode support
• Sync + async parity (async-first, sync via base class)

Requirements

• Valkey 9.1+ with valkey-search module loaded (v1.2+)
• valkey-glide>=2.3.1

Breaking Change Note

valkey-glide>=2.3.1 requires protobuf>=6.20. The current core dependency pins protobuf>=5.27.2,<6.0.0. This upper bound appears stale (no source code imports protobuf directly; it's a transitive dep of sentry-sdk/opentelemetry which both support protobuf 6.x+). The PR relaxes this to protobuf>=5.27.2.

Testing

24 smoke tests covering all BaseVectorDBProvider abstract methods, both happy and error paths, sync and async. Tested against Valkey 9.1.0-rc1 with search module v1.2.0.