test: Add resilience tests for MCP connection collapse and tool watcher fan-out

[towseef41]

Description

• Add resilience-focused unit tests: simulate MCP connection collapse to ensure _invoke_on_background_thread raises instead of hanging, and verify the tool watcher fans out file changes to all registries with a single observer.
• Reset shared ToolWatcher state between tests to avoid cross-test leakage.
• Clean up minor mypy complaints in _events.py tool/use accessors to keep lint passing.

Related Issues

• [FEATURE] Add tests for MCP connection collapse and tool watcher fan-out #1269

Documentation PR

• Not needed (test-only change).

Type of Change

• Test
• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Testing

• pre-commit run --all-files (includes lint/unit tests)
• 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