Revert "refactor: clean up iterateUntilDocuemntOrError"

This reverts commit e492067.

Author: baileympearson

test/tools/unified-spec-runner/operations.ts

@@ -309,15 +309,26 @@ operations.set('insertMany', async ({ entities, operation }) => {
 });
 
 operations.set('iterateUntilDocumentOrError', async ({ entities, operation }) => {
-  const cursorOrStream =
-    entities.getEntity('stream', operation.object, false) ??
-    entities.getEntity('cursor', operation.object, false);
+  function getChangeStream(): UnifiedChangeStream | null {
+    try {
+      const changeStream = entities.getEntity('stream', operation.object);
+      return changeStream;
+    } catch (e) {
+      return null;
+    }
+  }
 
-  if (cursorOrStream == null) {
-    throw new Error(`Unable to get entity - expect stream or cursor but received ${operation}`);
+  const changeStream = getChangeStream();
+  if (changeStream == null) {
+    // iterateUntilDocumentOrError is used for changes streams and regular cursors.
+    // we have no other way to distinguish which scenario we are testing when we run an
+    // iterateUntilDocumentOrError operation, so we first try to get the changeStream and
+    // if that fails, we know we need to get a cursor
+    const cursor = entities.getEntity('cursor', operation.object);
+    return await cursor.next();
   }
 
-  return await cursorOrStream.next();
+  return changeStream.next();
 });
 
 operations.set('listCollections', async ({ entities, operation }) => {