Feature/boto client config

[jona62]

Issue #, if available: N/A

Description of changes:

This PR enhances the MemorySessionManager with improved boto3 client configuration support and timestamp precision handling:

Commit 1: Add support for boto_client_config

• Added boto_client_config parameter to MemorySessionManager constructor to allow custom botocore configuration
• Implemented intelligent config merging that preserves user-provided settings while adding SDK user agent
• Enhanced user agent handling to append "bedrock-agentcore-sdk" to existing user agents or set as default
• Added comprehensive test coverage (331 new lines) including tests for:
  • Custom retry configurations
  • Connection pool settings
  • User agent preservation and merging
  • Integration with boto3 sessions
  • Edge cases and parameter validation

Commit 2: Override timestamp serializer for microsecond granularity

• Implemented custom timestamp serialization to preserve microsecond precision when sending datetime objects to AgentCore Memory service
• Refactored MemorySessionManager initialization into modular helper methods:
  • _validate_and_resolve_region() for region consistency validation
  • _build_client_config() for boto3 client configuration building
  • _configure_timestamp_serialization() for timestamp precision setup
• Fixed region validation logic to properly handle session region types
• Improved code organization and maintainability through method extraction

These changes provide:

1. Enhanced Configuration Flexibility: Users can now customize boto3 client behavior (retries, timeouts, connection pools) while maintaining SDK functionality
2. Improved Timestamp Precision: Datetime objects now preserve microsecond precision in API calls, critical for time-sensitive memory operations
3. Better Code Structure: Modular initialization methods improve readability and testability
4. Comprehensive Testing: Extensive test coverage ensures reliability across various configuration scenarios

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@72e37df). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/bedrock_agentcore/memory/session.py	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #98   +/-   ##
=======================================
  Coverage        ?   89.37%           
=======================================
  Files           ?       23           
  Lines           ?     2306           
  Branches        ?      331           
=======================================
  Hits            ?     2061           
  Misses          ?      140           
  Partials        ?      105           

Flag	Coverage Δ
unittests	89.37% <92.59%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.