fix(coderd/x/chatd): inflight wait group data race (#24007)

Addresses https://github.com/coder/internal/issues/1450

coderd/x/chatd/export_test.go

@@ -5,5 +5,5 @@ package chatd
 // database state only after asynchronous chat processing has completed.
 // Close waits for the same tracked work, but also stops the server.
 func WaitUntilIdleForTest(server *Server) {
-	server.inflight.Wait()
+	server.drainInflight()
 }