tests: improve typing

[DatGuy1]

• Corrected use of TypeVar (should be TypeAlias but only supported in 3.10+)
• Made text fields optional
• Check toastInput type more accurately

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 97.32%. Comparing base (6322266) to head (0ee8b93). Report is 10 commits behind head on main.

Files	Patch %	Lines
src/windows_toasts/toast_document.py	80.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #144 +/- ## ========================================== - Coverage 97.77% 97.32% -0.46% ========================================== Files 10 10 Lines 630 635 +5 Branches 90 92 +2 ========================================== + Hits 616 618 +2 - Misses 9 10 +1 - Partials 5 7 +2 ```

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