test(serdes): fix type ignore comment placement for lint

Move type: ignore comments to end of line for UnionType constructors
to satisfy pylint formatting requirements.

Verification results:
- All 309 serdes tests pass
- Full suite: 399 tests pass across all modules
- Lint: 9.98/10 rating (all sessions successful)
- mypy: Success, no issues found in 31 source files
- black: All files formatted correctly

Only new lint warning: C0302 (too many lines in module 3790/3000)
This is expected given we added 2195 lines of comprehensive tests.

Author: pavel-kirienko