feat(train and test sets): expose the paths to save train and test sets during L2G training

[project-defiant]

✨ Context

This PR adds the option to save the train and test sets from L2G training step.

🛠 What does this PR implement

• Add handlers to save train and test sets in the L2G Training
• Exposes training_set and test_set parameters to L2G Step configuration

🙈 Missing

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[Copilot]

Pull request overview

This PR extends the L2G (locus-to-gene) training step configuration so callers can optionally persist the generated train/test splits as parquet outputs during model training.

Changes:

• Added training_set and test_set output-path fields to LocusToGeneConfig.
• Wired training_set/test_set through LocusToGeneStep and implemented pandas-based parquet persistence after training.
• Added a config test asserting the new config fields exist.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/gentropy/config.py	Adds training_set and test_set fields to LocusToGeneConfig so they can be provided via structured config.
src/gentropy/l2g.py	Passes through new config params and writes train/test split pandas DataFrames to parquet when paths are provided.
tests/gentropy/test_config.py	Adds a unit test to ensure the new config fields are exposed on LocusToGeneConfig.

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