test: improve assertions in test_union_schema

[candleindark]

Summary

• Adds a test sub-case for nested union types (Union[int, Union[str, Bar1]]),
  verifying that nested unions are flattened correctly.
• Updates all other sub-cases in test_union_schema to assert the full
  SlotDefinition via == rather than checking individual attributes, making
  assertions more complete and consistent with the new nested-union case.
• For the tuple-choice sub-case, notes=slot.notes is used in the
  SlotDefinition comparison (instead of unittest.mock.ANY) because
  SlotDefinition.__eq__ serializes fields before comparing, which causes
  ANY to be treated as the literal string '<ANY>'. Note content is still
  verified separately with in_exactly_one_string.

Test plan

• hatch run test.py3.10:pytest tests/test_gen_linkml.py::TestSlotGenerator::test_union_schema

🤖 Generated with Claude Code