Add Valkey store support with lazy loading and comprehensive integration

[Copilot]

Adds complete support for Valkey, the open-source Redis fork, as a new key-value store implementation. This implementation provides full Redis compatibility while using the dedicated valkey-py client library.

Key Changes

New ValkeyStore Implementation

• Complete store implementation in src/kv_store_adapter/stores/valkey/ extending BaseManagedKVStore
• Multiple connection patterns supporting client injection, URL connections, and individual parameters:

  from kv_store_adapter import ValkeyStore
  
  # Direct parameters
  store = ValkeyStore(host="localhost", port=6379, db=0, password="secret")
  
  # URL connection  
  store = ValkeyStore(url="valkey://localhost:6379/0")
  
  # Existing client
  store = ValkeyStore(client=existing_valkey_client)

Package Integration & Lazy Loading

• Enhanced main package exports with __getattr__ lazy loading mechanism
• Clean import syntax for all stores: from kv_store_adapter import ValkeyStore
• Proper dependency handling with informative error messages for missing optional dependencies
• Updated pyproject.toml with valkey >= 6.0.0 optional dependency

Testing & Quality Assurance

• Comprehensive test suite following existing BaseStoreTests patterns
• Docker-based integration tests for real Valkey instance testing
• API consistency validation ensuring all 15 public methods match expected signatures
• Multiple initialization pattern testing covering all connection methods

Documentation Updates

• Added Valkey store section to DEVELOPING.md alongside existing Redis documentation
• Usage examples and configuration options matching established patterns
• Connection options documentation for all supported initialization methods

Technical Implementation Details

The ValkeyStore implements the same Redis-compatible API patterns:

• Uses valkey-py asyncio client for full async/await support
• Implements compound key management for collection-based organization
• Handles TTL operations with proper minimum value enforcement
• Provides JSON serialization through ManagedEntry objects
• Supports scan-based iteration for efficient large key set operations

This implementation maintains 100% API compatibility with existing store patterns while providing dedicated Valkey support as an alternative to Redis.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add valkey store support</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #8

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