docs: update memory READMEs with metadata types and message batching

[jariy17]

Summary

• Add documentation for EventMetadataFilter, StringValue, and MetadataValue types to the memory README
• Add batch_size parameter documentation to the Strands integration README
• Add new Message Batching section with context manager and explicit close() usage examples
• Add best practice note about flushing buffered messages when using batch_size > 1

These README updates were missed when #244 was merged.

Test plan

• Verify README renders correctly on GitHub
• Confirm documented types and parameters match the current codebase