fix(webapp): handle a nullish worker queue when deriving a run's region

Synthetic run snapshots (the mollifier buffer path) can carry a null or
undefined workerQueue, which their TaskRun casts hide from the type checker.
`baseWorkerQueue` now passes a nullish worker queue straight through instead
of calling `.indexOf` on it, so the replay loader and replay service no longer
throw when replaying a buffered run.

Author: ericallam

apps/webapp/app/runEngine/concerns/workerQueueSplit.server.ts

@@ -19,9 +19,16 @@ export const SCHEDULED_WORKER_QUEUE_SUFFIX = ":scheduled";
  * region is everything before the first `:`. Use this wherever a worker queue is
  * read as a region — for display, filtering, or as a region override — so
  * scheduled-split runs group under their real region instead of a phantom one.
- * Idempotent; returns the input unchanged when there's no suffix.
+ * Idempotent; returns the input unchanged when there's no suffix. A nullish
+ * worker queue (e.g. from a synthetic run snapshot) passes straight through.
  */
-export function baseWorkerQueue(workerQueue: string): string {
+export function baseWorkerQueue(workerQueue: string): string;
+export function baseWorkerQueue(workerQueue: string | null | undefined): string | null | undefined;
+export function baseWorkerQueue(workerQueue: string | null | undefined): string | null | undefined {
+  if (workerQueue == null) {
+    return workerQueue;
+  }
+
   const colon = workerQueue.indexOf(":");
   return colon === -1 ? workerQueue : workerQueue.slice(0, colon);
 }

apps/webapp/test/workerQueueSplit.test.ts

@@ -125,6 +125,11 @@ describe("baseWorkerQueue", () => {
     expect(baseWorkerQueue("")).toBe("");
   });
 
+  it("passes a nullish worker queue straight through (synthetic run snapshots)", () => {
+    expect(baseWorkerQueue(null)).toBeNull();
+    expect(baseWorkerQueue(undefined)).toBeUndefined();
+  });
+
   it("round-trips with workerQueueForRun: the split queue strips back to the region it came from", () => {
     const enqueued = workerQueueForRun({
       workerQueue: region,