Refactor LDC integration, add LDCHook provider, and include DAG structure test

- Added a new Airflow LDC provider under mokelumne/providers/ldc with provider.yaml and get_provider_info.py.
- Implemented LDCHook for authenticated LDC catalog access, including session creation, refresh handling, corpora page retrieval, and download response streaming.
- Refactored mokelumne/dags/fetch_ldc_corpus.py to use LDCHook for HTTP operations while keeping file writing in the DAG.
- Moved corpus filtering logic into mokelumne.util.ldc.filter_corpora and updated the DAG to consume parsed metadata lists.
- Added/updated unit tests for LDCHook, util LDC helpers, and DAG structure, including test_connection coverage.
- Updated imports for Airflow 3 compatibility and added public/ to .gitignore.

Author: anarchivist

.gitignore

@@ -15,4 +15,5 @@ airflow.cfg
 *.egg-info
 .coverage
 build/
+public/
 uv.lock