fix: make region optional in CodeInterpreter and code_session

[citizen204]

Summary

CodeInterpreter.__init__ and code_session() required an explicit region string, making callers responsible for extracting the region even when they already have a configured boto3 session or standard AWS environment variables in place.

This PR makes region optional (defaults to None). When omitted, the region is resolved from the boto3 session via its standard chain (environment variables, config file, instance metadata, etc.). The session is created first (if not supplied), then the region is read from session.region_name.

Existing callers passing an explicit region string are unaffected.

Fixes #511

Changes

• src/bedrock_agentcore/tools/code_interpreter_client.py:
  • CodeInterpreter.__init__: change region: str → region: Optional[str] = None; resolve region from session when not provided
  • code_session(): change region: str → region: Optional[str] = None
  • Update docstrings to reflect the new default behaviour

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@1eabc67). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...bedrock_agentcore/tools/code_interpreter_client.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage        ?   89.38%           
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  Files           ?       98           
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  Hits            ?     7671           
  Misses          ?      586           
  Partials        ?      325           

Flag	Coverage Δ
unittests	89.38% <33.33%> (?)

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