test(firestore): Add tests for forceIndex with "primary" (#8036)

Adds tests for `forceIndex` that use the `"primary"` index.
There was already a test that validates that `forceIndex` is passed to
the backend.

Author: dlarocque

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/PipelineTest.java

@@ -98,6 +98,7 @@
 import com.google.firebase.firestore.pipeline.AggregateHints;
 import com.google.firebase.firestore.pipeline.AggregateOptions;
 import com.google.firebase.firestore.pipeline.AggregateStage;
+import com.google.firebase.firestore.pipeline.CollectionGroupOptions;
 import com.google.firebase.firestore.pipeline.CollectionHints;
 import com.google.firebase.firestore.pipeline.CollectionSourceOptions;
 import com.google.firebase.firestore.pipeline.Expression;
@@ -4106,9 +4107,6 @@ public void testOptions() {
     assumeFalse(
         "Certain options are not supported against the emulator yet.", isRunningAgainstEmulator());
 
-    Pipeline.ExecuteOptions opts =
-        new Pipeline.ExecuteOptions().withIndexMode(Pipeline.ExecuteOptions.IndexMode.RECOMMENDED);
-
     double[] vector = {1.0, 2.0, 3.0};
 
     Pipeline pipeline =
@@ -4131,15 +4129,57 @@ public void testOptions() {
                 new AggregateOptions()
                     .withHints(new AggregateHints().withForceStreamableEnabled()));
 
+    // Should throw an error because there is no index named "abcdef".
+    // The error validates that the backend did attempt to use the index with the name specified
+    // in the forceIndex option.
     RuntimeException exception =
         assertThrows(
             RuntimeException.class,
             () -> {
               waitFor(pipeline.execute());
             });
+
     assertThat(exception.getMessage()).contains("Invalid index");
   }
 
+  @Test
+  public void testCollectionForceIndex() {
+    assumeFalse(
+        "Certain options are not supported against the emulator yet.", isRunningAgainstEmulator());
+
+    Task<Pipeline.Snapshot> execute =
+        firestore
+            .pipeline()
+            .collection(
+                randomCol,
+                new CollectionSourceOptions()
+                    .withHints(new CollectionHints().withForceIndex("primary")))
+            .limit(1)
+            .execute();
+
+    List<PipelineResult> results = waitFor(execute).getResults();
+    assertThat(results).hasSize(1);
+  }
+
+  @Test
+  public void testCollectionGroupForceIndex() {
+    assumeFalse(
+        "Certain options are not supported against the emulator yet.", isRunningAgainstEmulator());
+
+    Task<Pipeline.Snapshot> execute =
+        firestore
+            .pipeline()
+            .collectionGroup(
+                randomCol.getId(),
+                new CollectionGroupOptions()
+                    .withHints(new CollectionHints().withForceIndex("primary")))
+            .limit(1)
+            .execute();
+
+    List<PipelineResult> results = waitFor(execute).getResults();
+    assertThat(results).hasSize(1);
+  }
+
   @Test
   public void disallowDuplicateAliasesInAggregate() {
     IllegalArgumentException exception =