fix: correct MCPClient.__exit__ and stop() type annotations

[cogwirrel]

Description

Fix __exit__ and stop() parameter types to match the context-manager protocol: exc_type should be type[BaseException] | None (not BaseException), and all params on __exit__ need | None.

Related Issues

Fixes #2247

Type of Change

Bug fix

Testing

• uvx ty check passes (previously errored with invalid-context-manager)
• pytest tests/strands/tools/mcp/test_mcp_client.py — 62 passed
• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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