tests: add edge-case and validation tests

[Copilot]

Summary

Add 34 tests in tests/test_validation.py covering validation behavior:

• Missing required fields — ValidationError raised
• Extra unknown fields — silently ignored (serde default)
• Null values — ValidationError or ValueError depending on field type
• Empty strings — ValueError for Decimal, accepted for Str
• Boundary integers — no range validation (accepts negative, zero, max int)
• Malformed datetimes — ValidationError for invalid, date-only accepted
• Malformed decimals — ValueError, Infinity/NaN accepted per Python spec
• Nested validation — errors propagate from nested structures

Closes #33

Test plan

• All 34 new tests pass
• All existing tests still pass
• Linter and type checker pass