Feature: Add the ability to call a saved workflow from another

[JPPhoto]

Summary

This PR adds engine-native call_saved_workflow support. A parent workflow can select a saved workflow, bind values or connections to its exposed fields, suspend at the call node, execute the selected workflow as first-class child queue item(s), capture explicit named return values, and resume the parent workflow.

The implementation includes backend call-boundary execution, parent/child queue relationship metadata, access validation, dynamic input binding, bounded nested calls, strict child failure propagation, root-oriented retry behavior, child-row cancel/delete semantics, compatibility reporting, and named workflow returns via workflow_return_value, workflow_return, and workflow_return_get.

Related Issues / Discussions

This has been discussed many times on Discord.

QA Instructions

New tests in:

• tests/app/invocations/test_call_saved_workflows.py
• tests/app/services/test_workflow_call_batch.py
• tests/app/services/test_workflow_call_batch_runtime.py
• tests/app/services/test_workflow_call_compatibility.py
• tests/app/services/test_workflow_call_runtime.py
• tests/app/services/test_workflow_graph_builder.py
• tests/app/services/workflow_call_test_utils.py

Manual return-value test plan (note that this PR includes a database migration!):

1. Create a child workflow with integer, workflow_return_value, collect, and workflow_return.
2. Connect integer.value to workflow_return_value.value.
3. Set workflow_return_value.key to result.
4. Connect workflow_return_value.value to collect.item.
5. Connect collect.collection to workflow_return.values.
6. Save the child workflow.
7. Create a parent workflow with call_saved_workflow, workflow_return_get, and a downstream node such as add.
8. Select the saved child workflow in call_saved_workflow.
9. Connect call_saved_workflow.values to workflow_return_get.values.
10. Set workflow_return_get.key to result.
11. Connect workflow_return_get.value to add.a.
12. Set add.b to a literal value.
13. Run the parent workflow and confirm add.value equals the returned child value plus add.b.

For batch behavior, replace the child integer with integer_batch connected to integer.value, then return the integer.value through the same workflow_return_value path. The parent should receive result as a list of returned values.

Merge Plan

This branch includes a queue/session migration for workflow-call relationship metadata. Recheck migration ordering against other in-flight migrations before final merge.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[lstein]

Should this be tagged for 6.13?

[JPPhoto]

Should this be tagged for 6.13?

In an ideal world, with enough reviewers, sure. But realistically, the number of reviewers should be >1 and it should hit main just after 6.13 is released. This is a massive structural change and it should be in 6.14.