fix(executor): restore child-cost aggregation dropped by the staging merge

Staging's custom-block rewrite deleted `aggregateChildCost` from
workflow-handler.ts, and git merged that file cleanly — but this branch's
workflow-tool-runner.ts, added for the v2 execute migration, still imports it.
A silent semantic conflict: no marker, broken build.

Taking staging's rewrite is correct, so the helper is defined locally in its
one remaining consumer rather than resurrected in the file staging just
rewrote. Same four lines over the still-exported `calculateCostSummary`, so a
failed child workflow keeps billing the hosted-key spend it consumed instead
of reporting $0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: TheodoreSpeaks

apps/sim/executor/handlers/workflow/workflow-tool-runner.ts

@@ -1,22 +1,33 @@
 import { createLogger } from '@sim/logger'
 import { getErrorMessage } from '@sim/utils/errors'
 import { generateId } from '@sim/utils/id'
+import { calculateCostSummary } from '@/lib/logs/execution/logging-factory'
+import type { TraceSpan } from '@/lib/logs/types'
 import { ChildWorkflowError } from '@/executor/errors/child-workflow-error'
 import {
   buildCustomBlockExecutionContext,
   type CustomBlockExecutorContext,
 } from '@/executor/handlers/workflow/custom-block-tool-runner'
-import {
-  aggregateChildCost,
-  WorkflowBlockHandler,
-} from '@/executor/handlers/workflow/workflow-handler'
+import { WorkflowBlockHandler } from '@/executor/handlers/workflow/workflow-handler'
 import { classifyExecutionError } from '@/executor/utils/errors'
 import { parseJSON } from '@/executor/utils/json'
 import type { SerializedBlock } from '@/serializer/types'
 import type { ToolResponse } from '@/tools/types'
 
 const logger = createLogger('WorkflowToolRunner')
 
+/**
+ * Hosted-key spend of a failed child run, the way the parent bills it: recurse
+ * nested spans and de-dupe model breakdowns, then subtract the base execution
+ * charge the parent already applies once itself. A naive top-level `cost.total`
+ * sum undercounts when spend sits on nested children.
+ */
+function aggregateChildCost(childTraceSpans: TraceSpan[]): number {
+  if (childTraceSpans.length === 0) return 0
+  const summary = calculateCostSummary(childTraceSpans)
+  return Math.max(0, summary.totalCost - summary.baseExecutionCharge)
+}
+
 interface WorkflowToolParams {
   workflowId?: string
   inputMapping?: Record<string, unknown> | string