refactor: use `pydantic` for message validation

[lars-reimann]

Summary of Changes

Use pydantic to easily describe valid messages.

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	5	0	0	0.82s
✅ PYTHON	mypy	5		0	2.48s
✅ PYTHON	ruff	5	0	0	0.03s
✅ REPOSITORY	git_diff	yes		no	0.02s

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (e5e7011) to head (b935974).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #91 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 14 14 Lines 750 713 -37 ========================================= - Hits 750 713 -37 ```

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[lars-reimann]

:tada: This PR is included in version 0.13.0 :tada:

The release is available on:

• v0.13.0
• GitHub release

Your semantic-release bot :package::rocket: