get the region name from env if available

[afarntrog]

Description of changes:

This pull request updates the initialization logic for the AgentCoreMemorySessionManager to support setting the AWS region from the AWS_REGION environment variable, and adds a corresponding unit test to ensure this behavior works as expected.

Configuration improvements:

• Updated the __init__ method in session_manager.py to set region_name from the AWS_REGION environment variable if it is present, improving flexibility for deployment environments.

Testing enhancements:

• Added a new test test_init_with_aws_region_env_var in test_agentcore_memory_session_manager.py to verify that the session manager correctly uses the AWS_REGION environment variable during initialization.

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

[afarntrog]

╰─± uv run pre-commit run --all-files                        
uv-lock..................................................................Passed
ruff (legacy alias)......................................................Passed
ruff format..............................................................Passed
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check toml...............................................................Passed
check json...........................................(no files to check)Skipped
check for merge conflicts................................................Passed
check for added large files..............................................Passed
debug statements (python)................................................Passed
bandit...................................................................Passed
pytest with coverage.....................................................Passed

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@114a670). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #77   +/-   ##
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  Coverage        ?   88.70%           
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  Files           ?       20           
  Lines           ?     1859           
  Branches        ?      278           
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  Hits            ?     1649           
  Misses          ?      104           
  Partials        ?      106           

Flag	Coverage Δ
unittests	88.70% <100.00%> (?)

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