Fix issue#20603 (#13295)

Author: jialei-chen

mmv1/products/discoveryengine/SearchEngine.yaml

@@ -100,7 +100,6 @@ properties:
     description: |
       The data stores associated with this engine. For SOLUTION_TYPE_SEARCH type of engines, they can only associate with at most one data store.
     required: true
-    immutable: true
     item_type:
       type: String
   - name: 'createTime'

mmv1/third_party/terraform/services/discoveryengine/resource_discovery_engine_search_engine_test.go

@@ -1,9 +1,10 @@
 package discoveryengine_test
 
 import (
+	"testing"
+
 	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
 	"github.com/hashicorp/terraform-provider-google/google/acctest"
-	"testing"
 )
 
 func TestAccDiscoveryEngineSearchEngine_discoveryengineSearchengineBasicExample_update(t *testing.T) {
@@ -53,12 +54,21 @@ resource "google_discovery_engine_data_store" "basic" {
     solution_types              = ["SOLUTION_TYPE_SEARCH"]
     create_advanced_site_search = false
     }
+resource "google_discovery_engine_data_store" "second" {
+    location                    = "global"
+    data_store_id               = "tf-test-example2-datastore%{random_suffix}"
+    display_name                = "tf-test-structured-datastore2"
+    industry_vertical           = "GENERIC"
+    content_config              = "NO_CONTENT"
+    solution_types              = ["SOLUTION_TYPE_SEARCH"]
+    create_advanced_site_search = false
+    }
 resource "google_discovery_engine_search_engine" "basic" {
   engine_id = "tf-test-example-engine-id%{random_suffix}"
   collection_id = "default_collection"
   location = google_discovery_engine_data_store.basic.location
   display_name = "Example Display Name"
-  data_store_ids = [google_discovery_engine_data_store.basic.data_store_id]
+  data_store_ids = [google_discovery_engine_data_store.basic.data_store_id, google_discovery_engine_data_store.second.data_store_id]
   industry_vertical = google_discovery_engine_data_store.basic.industry_vertical
   common_config {
     company_name = "Example Company Name"
@@ -82,6 +92,15 @@ resource "google_discovery_engine_data_store" "basic" {
     solution_types              = ["SOLUTION_TYPE_SEARCH"]
     create_advanced_site_search = false
     }
+resource "google_discovery_engine_data_store" "second" {
+    location                    = "global"
+    data_store_id               = "tf-test-example2-datastore%{random_suffix}"
+    display_name                = "tf-test-structured-datastore2"
+    industry_vertical           = "GENERIC"
+    content_config              = "NO_CONTENT"
+    solution_types              = ["SOLUTION_TYPE_SEARCH"]
+    create_advanced_site_search = false
+    }
 resource "google_discovery_engine_search_engine" "basic" {
   engine_id = "tf-test-example-engine-id%{random_suffix}"
   collection_id = "default_collection"