fix: Remove client-side assertion for Union with subcollection and delegate to backend (#2356)

* Remove validation check

* allow the subquery test to run in prod env

* chore: generate libraries at Mon Apr 13 17:41:06 UTC 2026

---------

Co-authored-by: cloud-java-bot <cloud-java-bot@google.com>

Author: cherylEnkidu

google-cloud-firestore/src/main/java/com/google/cloud/firestore/Pipeline.java

@@ -1049,11 +1049,6 @@ public Pipeline sample(Sample sample) {
    * @return A new {@code Pipeline} object with this stage appended to the stage list.
    */
   public Pipeline union(Pipeline other) {
-    if (other.rpcContext == null) {
-      throw new IllegalArgumentException(
-          "Union only supports combining root pipelines, doesn't support relative scope Pipeline"
-              + " like relative subcollection pipeline");
-    }
     return append(new Union(other));
   }
 

google-cloud-firestore/src/test/java/com/google/cloud/firestore/it/ITPipelineSubqueryTest.java

@@ -68,9 +68,8 @@ List<Map<String, Object>> data(List<PipelineResult> results) {
   @Before
   public void setup() throws Exception {
     assumeFalse(
-        "This test suite only runs against the Enterprise edition in the Nightly environment.",
-        !getFirestoreEdition().equals(FirestoreEdition.ENTERPRISE)
-            || !"NIGHTLY".equals(getTargetBackend()));
+        "This test suite only runs against the Enterprise edition.",
+        !getFirestoreEdition().equals(FirestoreEdition.ENTERPRISE));
     if (collection != null) {
       return;
     }
@@ -984,20 +983,20 @@ public void testDirectExecutionOfSubcollectionPipeline() throws Exception {
 
   @Test
   public void testUnionWithSubqueryThrows() throws Exception {
-    IllegalArgumentException e =
+    ExecutionException e =
         assertThrows(
-            IllegalArgumentException.class,
+            ExecutionException.class,
             () -> {
               firestore
                   .pipeline()
                   .collection(collection.getPath())
-                  .union(PipelineSource.subcollection("subcollection"));
+                  .union(PipelineSource.subcollection("subcollection"))
+                  .execute()
+                  .get();
             });
 
-    assertThat(e)
-        .hasMessageThat()
+    assertThat(e.getCause().getMessage())
         .contains(
-            "Union only supports combining root pipelines, doesn't support relative scope Pipeline"
-                + " like relative subcollection pipeline");
+            "The 'subcollection(...)' stage can only be used at the start of a nested pipeline.");
   }
 }