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

Beginne damit, die drei Verweise auf MemoryClient in src/bedrock_agentcore/memory/README.md mit MemoryClient.__init__() in src/bedrock_agentcore/memory/client.py zu vergleichen. Prüfe die drei vorgeschlagenen Optionen und kläre die beabsichtigte Deprecation Policy mit einem Maintainer, bevor du Änderungen vornimmst. Die Aufgabe ist erledigt, wenn die Dokumentation und jedes genehmigte Warnverhalten den Status von MemoryClient konsistent beschreiben.

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