feat(memory): add boto3_session parameter to MemoryClient

[jariy17]

Summary

• Add optional boto3_session parameter to MemoryClient.__init__(), enabling named profiles, custom credentials, and other session-level configuration
• Consistent with MemorySessionManager which already accepts boto3_session
• Updated all tests to mock boto3.Session instead of boto3.client to match the new session-based client creation

Closes #44
Based on #90 by @manikawnth

Test plan

• All 112 existing + new unit tests pass
• New tests for boto3_session parameter (with and without explicit region_name)