fix(firestore): ensure limit(0) is properly serialized in query requests (#8076)

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/src/reference/query-util.ts

@@ -344,7 +344,7 @@ export class QueryUtil<
                   // call to `requestStream()` will backoff should the restart
                   // fail before delivering any results.
                   let newQuery: Query<AppModelType, DbModelType>;
-                  if (!this._queryOptions.limit) {
+                  if (this._queryOptions.limit === undefined) {
                     newQuery = query;
                   } else {
                     const newLimit =

handwritten/firestore/dev/src/reference/query.ts

@@ -1608,7 +1608,7 @@ export class Query<
     structuredQuery.startAt = this.toCursor(this._queryOptions.startAt);
     structuredQuery.endAt = this.toCursor(this._queryOptions.endAt);
 
-    if (this._queryOptions.limit) {
+    if (this._queryOptions.limit !== undefined) {
       structuredQuery.limit = {value: this._queryOptions.limit};
     }
 

handwritten/firestore/dev/test/query.ts

@@ -1957,6 +1957,20 @@ describe('limit() interface', () => {
     query = query.limit(1).limit(2).limit(3);
     await query.get();
   });
+
+  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', () => {