Declare uns['modality'] in the task file formats

[rcannood]

Describe your changes

process_dataset writes uns["modality"] on all four outputs, and quite a lot of the repo depends on it:

• run_benchmark/main.nf -- picks which object's normalization_id to trust (dataset_uns_mod1.modality == "GEX")
• guanlab_dengkw_pm -- keys n_comp_dict on (mod1_type, mod2_type)
• cellmapper_scvi -- chooses scVI vs PoissonVI
• novel_train / novel_predict -- pick the model class
• simple_mlp_train / simple_mlp_predict -- build the GEX2ADT-style task name

But it appears in none of file_train_mod{1,2}.yaml / file_test_mod{1,2}.yaml. Anyone writing a new data processor against the published contract would satisfy the spec and break six methods -- and run_benchmark would quietly fall through to the mod2 branch, since null == "GEX" is just false.

Declaring it as a required uns string. Verified all 16 files under resources_test/task_predict_modality/ already carry it (GEX/ADT/ATAC), so nothing is excluded.

Part of a series of PRs coming out of a pre-run review of the benchmark.

Checklist before requesting a review

• I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • Breaking changes
  • New functionality
  • Major changes
  • Minor changes
  • Bug fixes

• Proposed changes are described in the CHANGELOG.md

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