fix(billing): release reserved slot on execute-route 503 and setup throw

After preprocessExecution reserves a billing concurrency slot, the streaming
path could exit without releasing it: the 503 return when
initializeExecutionStreamMeta fails, and any throw during stream setup (caught
by the outer handler, which only returned 500). Both left the slot held until
TTL, wrongly throttling unrelated runs. Release on the 503 path and in the
outer catch (executionId hoisted so the catch can see it; release is
idempotent and a no-op when no slot was reserved).

Author: waleedlatif1

apps/sim/app/api/workflows/[id]/execute/route.ts

@@ -378,6 +378,10 @@ async function handleExecutePost(
   }
   const callChain = buildNextCallChain(incomingCallChain, workflowId)
 
+  // Hoisted so the outer catch can release a reserved billing slot when a throw
+  // after preprocessExecution exits before the stream takes over its release.
+  let executionId = ''
+
   try {
     const auth = await checkHybridAuth(req, { requireWorkflowId: false })
     const isMcpBridgeRequest =
@@ -637,8 +641,7 @@ async function handleExecutePost(
       )
     }
 
-    const executionId =
-      isClientSession && requestedExecutionId ? requestedExecutionId : generateId()
+    executionId = isClientSession && requestedExecutionId ? requestedExecutionId : generateId()
     reqLogger = reqLogger.withMetadata({ userId, executionId })
 
     reqLogger.info('Starting server-side execution', {
@@ -1166,6 +1169,7 @@ async function handleExecutePost(
     })
     if (!metaInitialized) {
       timeoutController.cleanup()
+      await releaseExecutionSlot(executionId)
       return NextResponse.json(
         { error: 'Run buffer temporarily unavailable' },
         { status: 503, headers: { 'X-Execution-Id': executionId } }
@@ -1724,6 +1728,9 @@ async function handleExecutePost(
     })
   } catch (error: any) {
     reqLogger.error('Failed to start workflow execution:', error)
+    // Release a reserved billing slot if a throw exited before the stream took
+    // over its release (idempotent; no-op when never reserved).
+    if (executionId) await releaseExecutionSlot(executionId)
     return NextResponse.json(
       { error: error.message || 'Failed to start workflow execution' },
       { status: 500 }