Fix/python 3.14 test exclusions

[Exudev]

This PR stabilizes the Hamilton test suite for development and CI on Python 3.14 by excluding currently incompatible plugins from the collection phase and resolving noisy configuration warnings.

Changes

• Core Test Exclusions: Updated tests/conftest.py to add several plugins (Pandera, DLT, Pydantic, etc.) to the collect_ignore list for Python >= 3.14. These plugins currently cause collection-time crashes on 3.14 due to upstream dependency delays.
• Pytest Configuration: Formally registered the asyncio marker in pyproject.toml. This resolves hundreds of PytestUnknownMarkWarning warnings that previously cluttered the test logs.
• Plugin Test Cleanup: Updated plugin_tests/h_dask/conftest.py to include 3.14 exclusion logic, ensuring consistency across the repository.

How I tested this

Tested locally on Python 3.14.2:

• Ran the full test suite using ./venv/bin/pytest tests.
• Result: Successfully transitioned from 10 fatal collection ERRORS to 1,294 passing tests (0 failures, 5 skipped).
• Verified that pytest-asyncio warnings are now resolved.

Notes

• These exclusions should be revisited as the plugin ecosystem (Pandera, Pydantic, etc.) releases official support for Python 3.14.
• This PR follows the same pattern recently established for Ray and Polars exclusions.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested (Test suite health verified)
• New functions are documented (N/A - maintenance PR)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

Fixes #1586

[Exudev]

@skrawcz