(#2567) fix htmx renderer for checkpoint selection

[bghira]

This pull request enhances the handling and population of options for the resume_from_checkpoint field in the form, ensuring that available checkpoints in the specified output directory are accurately listed and selectable. It also improves the robustness of the front-end logic for detecting the output directory input and adds a test to verify the new behavior.

Backend improvements to checkpoint option handling:

• Added logic to the FieldService to dynamically populate the resume_from_checkpoint field's options with available checkpoints from the specified output directory, ensuring deduplication and inclusion of the current value if not already present.
• Imported CheckpointManager to support checkpoint discovery in the backend service.

Frontend robustness:

• Updated JavaScript in form_field.html to reliably detect the output directory input field by checking multiple possible selectors, improving compatibility with different form layouts. [1] [2]

Testing:

• Added a unit test (test_field_service_resume_from_checkpoint.py) to verify that local checkpoints are correctly included in the options for the resume_from_checkpoint field.