fix: support deferred class annotations on Python 3.14

[Excelius-Wang]

Summary

• resolve dataclass annotations from the created class instead of the class-body namespace
• use Format.FORWARDREF on Python 3.14 to support unresolved and self-referencing annotations
• preserve compatibility with Python 3.8–3.13
• handle both classes and instances passed to to_dict(classes=...)

Closes #1974

Testing

• Python 3.14.6: import unitxt
• Python 3.14.6: python -m unittest tests.library.test_dataclass — 21 passed
• Python 3.12.4: python -m unittest tests.library.test_dataclass — 20 passed, 1 skipped
• simulated Python 3.8/3.9 annotation-inspection path — 20 passed, 1 skipped
• pre-commit run --files src/unitxt/dataclass.py tests/library/test_dataclass.py