Migrate os.path to pathlib.Path in file_processor_handler.py

[Dotify71]

Description

Fixes #68757

The airflow.utils.log.file_processor_handler module currently relies heavily on the os.path library for filesystem path manipulations. To modernize the codebase, improve cross-platform readability, and align with modern Python standards, this PR migrates os.path calls (e.g., os.path.join, os.path.abspath, os.path.isdir) to their pathlib.Path equivalents where possible.

Testing

• Existing Airflow unit tests will cover these pathways as this handler is central to DAG parsing logging.

[pierrejeambrun]

LGTM overall, just one comment though

[potiuk]

Mixed unrelated changes.

[Dotify71]

Thanks @potiuk for the review!

I have:

1. Dropped the unrelated Alembic patch commit.
2. Fixed the failing static checks (removed the unused import os in file_processor_handler.py and changed .resolve() to .absolute() to match the original os.path.abspath behavior, which doesn't resolve symlinks and was breaking tests).

This is now a clean 1-commit PR. Let me know if anything else is needed!