dev: mypy reports missing type stubs and import errors during type checking

[SashankBhamidi]

Running mypy on the codebase produces multiple import warnings.

1. Missing type stubs for icalendar library

tests/conftest.py:5: error: Library stubs not installed for "icalendar" [import-untyped]
tests/test_multiple_vcalendars.py:10: error: Library stubs not installed for "icalendar" [import-untyped]

Mypy cannot provide proper type checking for the core icalendar library used throughout the codebase.

2. Missing type stubs for typer.testing

tests/conftest.py:7: error: Cannot find implementation or library stub for module named "typer.testing" [import-not-found]

The CLI testing utilities lack type information.

3. Local module import resolution issues

tests/conftest.py:9: error: Cannot find implementation or library stub for module named "mergecal" [import-not-found] 
tests/test_multiple_vcalendars.py:7: error: Cannot find implementation or library stub for module named "mergecal" [import-not-found]

Mypy cannot resolve imports to the local mergecal package during test runs.