Configure mypy to respect gitignore

[adamtheturtle]

Summary

• configure mypy with exclude_gitignore = true
• prevent files ignored by Git from being included in mypy discovery

Impact

Mypy will respect the project's .gitignore rules when discovering files. This avoids type-checking generated, local, or otherwise ignored files.

Validation

• formatted and parsed pyproject.toml with pyproject-fmt
• verified the branch diff is exactly one added line in pyproject.toml
• ran Git whitespace validation

[adamtheturtle]

Closing this duplicate; #2452 contains the same branch and change.