Attach executables and a dataset to a workunit in create_workunit

[leoschwarz]

• create_workunit accepts executables (name → base64 content), attached to the workunit as context: "WORKUNIT" — e.g. the script that generated it.
• create_workunit accepts dataset (name + base64 csv/tsv/parquet), created as the workunit's output dataset via the existing create_dataset.
• create_workunit accepts input_dataset_id, referencing an existing dataset as the workunit's input (inputdatasetid, previously only ever read).
• params may now be a plain mapping instead of a CreateWorkunitParams; invalid input raises ValidationError before any write.
• Both new steps run inside the existing failure cleanup, so a dataset failure marks the workunit failed rather than leaving it available.
• POST /create/workunit/v1 gains all three fields, inherited via CreateWorkunitRequest.

🤖 Prepared with assistance from Claude Opus 5 via Claude Code.