fix(workflow): clear nextStepIds when converting a step to If/Else (#23714)

## Context

Fixes #22947.

A workflow with If/Else branches could fail at runtime with `Step not
found` (and no detail in the Runs panel) because of dangling
`nextStepIds` in the workflow graph — references to steps that no longer
exist.

## Root cause

An If/Else routes only through `settings.input.branches[].nextStepIds`;
its top-level `nextStepIds` is never read by the executor and must stay
empty. But converting an existing step into an If/Else copied the
previous step's `nextStepIds` onto the new If/Else, leaving a stray
top-level reference. That reference is invisible to the executor, and
when steps around it are later deleted it becomes dangling and
propagates into a normal step's `nextStepIds`, which the executor then
tries to follow — `Step not found`.

## Fix

When a step's type is changed to If/Else, don't carry over the previous
step's `nextStepIds`. One change in
`workflow-version-step-update.workspace-service.ts`.

## Verification

Reproduced on a local instance via the editor's GraphQL mutations (build
`trigger → P → X → D`, convert X to If/Else, delete D then X):
- Before: converting X produced a stray `nextStepIds: [D]`, and after
the deletes P was left with a dangling reference.
- After: converting X yields `nextStepIds: []`, and P stays clean — no
dangling reference.

packages/twenty-server/src/modules/workflow/workflow-builder/workflow-version-step/utils/get-next-step-ids-for-step-type-change.util.ts

@@ -0,0 +1,16 @@
+import { isWorkflowIfElseAction } from 'src/modules/workflow/workflow-executor/workflow-actions/if-else/guards/is-workflow-if-else-action.guard';
+import { type WorkflowAction } from 'src/modules/workflow/workflow-executor/workflow-actions/types/workflow-action.type';
+
+export const getNextStepIdsForStepTypeChange = ({
+  existingStep,
+  builtStep,
+}: {
+  existingStep: WorkflowAction;
+  builtStep: WorkflowAction;
+}): string[] | undefined => {
+  if (isWorkflowIfElseAction(builtStep)) {
+    return [];
+  }
+
+  return existingStep.nextStepIds;
+};

packages/twenty-server/src/modules/workflow/workflow-builder/workflow-version-step/workflow-version-step-update.workspace-service.ts

@@ -8,6 +8,7 @@ import {
   WorkflowVersionStepExceptionCode,
 } from 'src/modules/workflow/common/exceptions/workflow-version-step.exception';
 import { WorkflowSchemaWorkspaceService } from 'src/modules/workflow/workflow-builder/workflow-schema/workflow-schema.workspace-service';
+import { getNextStepIdsForStepTypeChange } from 'src/modules/workflow/workflow-builder/workflow-version-step/utils/get-next-step-ids-for-step-type-change.util';
 import { WorkflowVersionStepHelpersWorkspaceService } from 'src/modules/workflow/workflow-builder/workflow-version-step/workflow-version-step-helpers.workspace-service';
 import { WorkflowVersionStepOperationsWorkspaceService } from 'src/modules/workflow/workflow-builder/workflow-version-step/workflow-version-step-operations.workspace-service';
 import { type WorkflowAction } from 'src/modules/workflow/workflow-executor/workflow-actions/types/workflow-action.type';
@@ -133,7 +134,10 @@ export class WorkflowVersionStepUpdateWorkspaceService {
         step: {
           ...builtStep,
           id: existingStep.id,
-          nextStepIds: existingStep.nextStepIds,
+          nextStepIds: getNextStepIdsForStepTypeChange({
+            existingStep,
+            builtStep,
+          }),
           position: existingStep.position,
         },
         workspaceId,