Batch processing fails when items come from shell node JSON output

[spinje]

Summary

When using batch.items with output from a shell node that produces a JSON array, the batch processor fails with:

TypeError: Batch items must be an array, got str.
Template '${create-array.stdout}' resolved to: '["item1", "item2", "item3"]'

Root Cause

Shell nodes always output text (stdout is a string). When that text is a valid JSON array like ["a", "b", "c"], the batch processor receives it as a string, not a parsed Python list.

The node parameter system (node_wrapper.py) already handles this case - it auto-parses JSON strings when the target parameter expects dict or list. But batch.items doesn't have this auto-parsing.

Reproduction

{
  "nodes": [
    {
      "id": "create-array",
      "type": "shell",
      "params": {"command": "echo '[\"item1\", \"item2\", \"item3\"]'"}
    },
    {
      "id": "process-items",
      "type": "shell",
      "batch": {"items": "${create-array.stdout}"},
      "params": {"command": "echo Processing: ${item}"}
    }
  ],
  "edges": [{"from": "create-array", "to": "process-items"}]
}

Expected: Batch processes each item (item1, item2, item3)
Actual: TypeError because ${create-array.stdout} is a string, not a list

Real-World Use Case

This blocks a useful parallelization pattern:

1. Shell node with jq splits HTML into sections: cat file.html | jq -R -s 'split("<section>")'
2. Batch LLM node processes each section in parallel
3. Results aggregated

Currently this fails because jq outputs a JSON array as text.

Proposed Solution

Add JSON auto-parsing in batch_node.py:prep() method, following the same pattern as node_wrapper.py:746-781:

1. Check if resolved items is a string
2. Strip whitespace (shell outputs often have trailing \n)
3. If it starts with [, attempt json.loads()
4. If parsing succeeds and result is a list, use it
5. Otherwise, keep as string and let existing error handling work

This makes batch.items consistent with how node params already handle JSON strings.

Files to Modify

• src/pflow/runtime/batch_node.py - Add JSON auto-parsing in prep() method
• tests/test_runtime/test_batch_node.py - Add tests for JSON string items