Warn when overwriting existing loader in register_loader

[swagat-mishra28]

Closes #903

Summary

While reviewing the loader registration mechanism in movement/io/load.py, I noticed that
register_loader() silently overwrites existing entries in the _LOADER_REGISTRY
when a loader is registered multiple times for the same source_software.

This behaviour can make debugging difficult, as an existing loader may be replaced
without any indication.

This PR improves transparency by emitting a warning when a loader is being
overwritten, while preserving the current behaviour that allows overwriting.

Changes

• Added a warning in register_loader() when a loader for the same
  source_software already exists in _LOADER_REGISTRY.
• The loader is still overwritten to maintain backward compatibility.

Example:

if source_software in _LOADER_REGISTRY:
    logger.warning(
        f"Loader for '{source_software}' is being overwritten."
    )

Testing

All tests were executed after the change:

pytest --ignore=tests/test_unit/test_napari_plugin

Result:

1011 passed, 4 deselected, 2 xfailed

Please let me know if you would prefer raising an error instead of a warning for duplicate loader registrations.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud