Add MyPy coverage and enable type checking in CI

[google-labs-jules]

This PR adds MyPy to the project to enforce static type checking.

Changes include:

• Adding [tool.mypy] to pyproject.toml with ignore_missing_imports and check_untyped_defs.
• Fixing trivial type errors such as missing import unittest in test files and incorrect usage of os.sys in examples.
• Fixing a type error in flask_cors/core.py where headers was initialized as MultiDict but MyPy couldn't infer the type correctly without a hint, and handling None in expose_headers.
• Enabling the MyPy check in .github/workflows/main.yml.
• Updating the Makefile to include the mypy check in make check.

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PR created automatically by Jules for task 3985676042734546971 started by @corydolphin

[google-labs-jules]

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