Tests

Enable mypy strict-optional and fix nullability gaps (closes #69) (#79) #131

Job	Run time
Secret scan (gitleaks)	4s
Lock file freshness	8s
Typecheck (mypy)	18s
Unit tests (ubuntu-latest / Python 3.13)	14s
Unit tests (ubuntu-latest / Python 3.12)	19s
Unit tests (ubuntu-latest / Python 3.10)	12s
Unit tests (macos-latest / Python 3.11)	12s
Unit tests (macos-latest / Python 3.13)	12s
Unit tests (windows-latest / Python 3.10)	1m 3s
Unit tests (windows-latest / Python 3.11)	1m 17s
Unit tests (ubuntu-latest / Python 3.11)	16s
Unit tests (windows-latest / Python 3.12)	1m 28s
Unit tests (macos-latest / Python 3.12)	12s
Unit tests (macos-latest / Python 3.10)	41s
Unit tests (windows-latest / Python 3.13)	46s
	7m 22s

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