feat: add shell tool and tests

[notowen333]

Issue #, if available:
https://github.com/strands-agents/private-sdk-python-staging/issues/340

Description of changes:

• simple subprocess based shell avoiding full terminal emulation with pty

Distinct from TS:

• persistent shell (i.e. cd and export will persist across tool calls)
• binary encoded output is parsed
• standard error and standard out write to the same pipe to avoid race conditions
• clean garbage collection tied to agent lifecycle
• uses default shell instead of specifying bash

First PR to add pytest tests to the python scripts section of the repo, so needed to add __init__ and requirements.txt

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