#37036 feat(Execute Sub-workflow Node)!: Remove "Run once for each item" mode

Summary

Removes the "Run once for each item" mode from the Execute Sub-workflow node, per the v3 breaking-changes tracker. This mode does not behave correctly in combination with waiting, and a Loop Over Items node achieves the same behavior.

• Removes the Mode selection from the node panel and adds node version 1.4. The node always runs once with all items.
• On versions 1 to 1.3 — the versions where the mode could legitimately be set — the mode parameter is kept as a hidden property, gated to @version <= 1.3. This is deliberate: Workflow construction rebuilds node.parameters from the declared and displayed properties only, so without the declaration a stale each value would be stripped and the workflow would silently run as once, changing what data the sub-workflow receives. The hidden declaration lets the stored value survive so the node can fail loudly instead.
• A workflow saved with mode: 'each' on version <= 1.3 shows a warning notice in the editor and fails at runtime with: The "Run once for each item" mode is no longer available, instructing the user to replace the node and add a "Loop Over Items" node before it.
• On version 1.4, mode is not declared. A value injected via the API or workflow JSON is stripped during parameter normalization and the node runs once with all items — the standard n8n treatment for parameters that are not part of a node version's contract.
• The v3 breaking-change detection rule (execute-workflow-each-mode-v3) and the deprecation notice already shipped on master; this PR is only the removal.

Behavior overview

Node version	mode: 'each' present	Behavior
<= 1.3	yes	Editor warning + runtime failure with migration instructions
<= 1.3	no	Runs once with all items
1.4	injected via API/JSON	Value stripped, runs once with all items
1.4	no	Runs once with all items

How to test

1. On a current release build, create a workflow: Manual Trigger → Execute Sub-workflow with Mode set to "Run once for each item", pointing at any sub-workflow.
2. Start n8n from this branch.
3. Open the node: no Mode field renders, and a warning notice explains that the node uses the removed mode and how to migrate.
4. Execute the workflow: the node fails with the removal error and migration instructions.
5. Save the workflow without changes and execute again: it must still fail (the hidden declaration preserves the stale value through save-time parameter normalization).
6. Create a new Execute Sub-workflow node: it is created at version 1.4 with no Mode field and runs once with all items.

Unit tests cover the removed-mode failure and the once paths.

Related Linear tickets, Github issues, and Community forum posts

https://linear.app/n8n/issue/CAT-4214

Review / Merge checklist

• I have seen this code, I have run this code, and I take responsibility for this code.
• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with Backport to Beta, Backport to Stable, or Backport to v1 (if the PR is an urgent fix that needs to be backported)

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