Add unit test for MLflow tracking URI regression fix

[Copilot]

Change Description

Adds regression test for the MLflow tracking directory fix from PR #693. The original fix ensured MLflow stores its backend in the root results directory rather than per-run subdirectories, but lacked test coverage.

Solution Description

Test Implementation

• Mocks mlflow.set_tracking_uri to verify it receives file://<root_results_dir>/mlflow
• Validates fix using existing loopback_hyrax fixture (both dataset variants)
• Placed in tests/hyrax/test_train.py alongside existing training tests

Additional Changes

• Added mlflow.db to .gitignore to exclude test-generated database files

Code Quality

• I have read the Contribution Guide and agree to the Code of Conduct
• My code follows the code style of this project
• My code builds (or compiles) cleanly without any errors or warnings
• My code contains relevant comments and necessary documentation

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