Support image columns and media collection in multi-shard Slurm runs #1114

Problem

Slurm planning explicitly rejects image columns when array_tasks.count > 1 in packages/data-designer-slurm/src/data_designer/slurm/planning/resolution.py.

Image generation can persist media separately from dataset rows. The current collection implementation merges verified Parquet outputs; it does not consolidate shard-local media files and preserve or rewrite their references. Removing the planning guard alone would therefore leave disk-backed image outputs unsupported.

Proposed solution

Define and implement image-output handling across shard generation, retries, winner publication, and collection. Include media artifacts in the output contract as needed, give them collision-safe identities, and ensure collected rows resolve to the correct images. Specify which persistence modes are supported.

Acceptance criteria

• A dataset with an image column can run across multiple array tasks and be collected successfully.
• Collected media references remain valid after collection, without depending on temporary allocation storage.
• Media names cannot collide across shards or attempts.
• Collection uses artifacts from winning attempts only and validates required files and their integrity.
• Missing or corrupt media produces an actionable failure; retries and repeated collection do not publish partial output.
• Tests cover supported persistence modes, shard retries, artifact references, and collection; documentation states any remaining restrictions.

Alternatives

Use a single array task for image generation, or assemble media and dataset references manually after separate runs.

Context

Related to #850. Plugin/custom-column sharding is tracked separately.