Clarify MemoryClient deprecation status - documentation inconsistency

[jariy17]

Summary

The documentation for MemoryClient has inconsistent messaging about its status:

1. 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: "While still supported, we recommend migrating to MemorySessionManager for new projects"
3. 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.

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