Tests

Enable mypy strict-optional and fix nullability gaps (closes #69) #127

Job	Run time
Unit tests (windows-latest / Python 3.10)	1m 25s
Unit tests (macos-latest / Python 3.10)	37s
Unit tests (macos-latest / Python 3.11)	15s
Unit tests (ubuntu-latest / Python 3.11)	11s
Unit tests (ubuntu-latest / Python 3.13)	13s
Secret scan (gitleaks)	6s
Unit tests (windows-latest / Python 3.12)	1m 34s
Unit tests (macos-latest / Python 3.12)	12s
Unit tests (windows-latest / Python 3.11)	1m 33s
Typecheck (mypy)	18s
Unit tests (macos-latest / Python 3.13)	15s
Unit tests (windows-latest / Python 3.13)	1m 44s
Lock file freshness	10s
Unit tests (ubuntu-latest / Python 3.10)	16s
Unit tests (ubuntu-latest / Python 3.12)	18s
	9m 7s

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