fix(firestore): ensure limit(0) is properly serialized in query requests

Previously, calling .limit(0) on a Firestore Query did not serialize
the zero value to the wire protocol, causing queries to be sent without
a limit constraint. This change explicitly includes limit(0) in the
query proto.

Fixes #7382

Author: liu-husong

handwritten/firestore/dev/test/query.ts

@@ -1957,6 +1957,23 @@ describe('limit() interface', () => {
     query = query.limit(1).limit(2).limit(3);
     await query.get();
   });
+
+  // Regression test: This test currently fails because limit(0) is not
+  // serialized in the query proto (limit(0) is falsy in JavaScript).
+  // This test is expected to fail until the fix is applied.
+  it('handles limit(0) correctly', async () => {
+    const overrides: ApiOverride = {
+      runQuery: request => {
+        queryEquals(request, limit(0));
+        return emptyQueryStream();
+      },
+    };
+
+    firestore = await createInstance(overrides);
+    let query: Query = firestore.collection('collectionId');
+    query = query.limit(0);
+    await query.get();
+  });
 });
 
 describe('limitToLast() interface', () => {