Clarify MemoryClient deprecation status - documentation inconsistency

## Summary

The documentation for `MemoryClient` has inconsistent messaging about its status:

1. [`src/bedrock_agentcore/memory/README.md#L39`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/README.md#L39): "MemoryClient - Legacy client interface (still supported but not recommended for new projects)"
2. [`src/bedrock_agentcore/memory/README.md#L120`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/README.md#L120): "While still supported, we recommend migrating to MemorySessionManager for new projects"
3. [`src/bedrock_agentcore/memory/README.md#L555`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/README.md#L555): "MemoryClient: Legacy client interface (deprecated)"

The first two say "still supported" while the third says "deprecated". Additionally, there's no actual `DeprecationWarning` in the code when users instantiate [`MemoryClient`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/client.py#L43).

## Proposed Options

### Option A: Formally deprecate MemoryClient
- Add `DeprecationWarning` to `MemoryClient.__init__()`
- Update all documentation to consistently say "deprecated"
- Set a target removal version (e.g., v2.0.0)

### Option B: Keep as "legacy" without formal deprecation
- Remove "deprecated" from API Reference section
- Keep "legacy" and "not recommended" language
- No code changes needed

### Option C: Staged approach
- Fix documentation to be consistent ("legacy, not recommended")
- Add deprecation warning in a future minor release
- Remove in a future major release

Commencez par comparer les trois références à MemoryClient dans src/bedrock_agentcore/memory/README.md avec MemoryClient.__init__() dans src/bedrock_agentcore/memory/client.py. Examinez les trois options proposées et confirmez la politique de dépréciation prévue avec un maintainer avant toute modification. La tâche est terminée lorsque la documentation et tout comportement d’avertissement approuvé décrivent de manière cohérente le statut de MemoryClient.

Rédigé par le modèle d'indexation à partir du texte de l'issue.