feat: add 3.15 support to pyarrow packages

[daniel-sanche]

Now that pyarrow has published Python 3.15 wheels on PyPI, this PR adds the Python 3.15 classifier to the remaining pyarrow-dependent libraries and removes the Python 3.15 unit test skips.

Closes #18535
Part of #17741
Fixes #18532

BEGIN_COMMIT_OVERRIDE
feat: support Python 3.15
END_COMMIT_OVERRIDE

[gemini-code-assist]

Code Review

This pull request enables testing and official support for Python 3.15 across several packages, including bigquery-magics, db-dtypes, google-cloud-bigquery-storage, google-cloud-bigquery, google-cloud-documentai-toolbox, pandas-gbq, and sqlalchemy-bigquery. This is achieved by removing the Python 3.15 test-skipping logic from the noxfile.py configurations and adding the Python 3.15 classifier to the setup.py files. I have no feedback to provide as the changes are straightforward and correct.