Python: [Feature]: Add Valkey Context Provider and Chat Message Store

@westey-m is already working on this.

Since Jun 4, 2026.

### Description

### Problem
The current `agent-framework-redis` package depends on `redisvl>=0.8.2`, which requires Redis Stack's RediSearch module for the `RedisContextProvider`'s vector operations. While Redis and Valkey are mostly protocol-compatible for core features, their search modules are not compatible — code that requires RediSearch will not work with Valkey's valkey-search module. Additionally, the RedisVL library itself has known incompatibilities with Valkey. Since this dependency is bundled at the package level, even the `RedisChatMessageStore` (which only needs basic key-value operations) pulls in RedisVL.

This is a practical blocker for teams running Valkey in production — whether self-hosted or through managed cloud services — which has become increasingly common since the Redis license change.

### Proposed Solution

Add a new `agent-framework-valkey` package providing:

1. **`ValkeyChatMessageStore`** — persistent chat message storage implementing `ChatMessageStoreProtocol`, using `valkey-glide` (the official Valkey Python client) for basic key-value operations.

2. **`ValkeyContextProvider`** — long-term memory context provider implementing the `ContextProvider` protocol, using `valkey-glide` with Valkey's native vector search capabilities for semantic retrieval of past conversation context.

The package would follow the same structure and patterns as `agent-framework-redis` and `agent-framework-mem0`.

#### Value

- Performance: Valkey's valkey-search module delivers single-digit millisecond latency with 99%+ recall for vector search operations
- Scaling: Linear scaling with cluster mode support
- Open-source governance: Linux Foundation project with community-driven development, clear licensing
- Cloud support: Managed services from major cloud providers.
- Migration path: Easy migration from Redis deployments for teams affected by the Redis license change

### Code Sample

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### Language/SDK

Python