feat(net-lb-app-int): support regional health checks (#3939)

Introduce support for regional health checks in the net-lb-app-int module while maintaining backward compatibility.

Added optional is_regional flag to health_check_configs (defaulting to false). When true, it creates google_compute_region_health_check instead of google_compute_health_check.

Updated backend services and outputs to merge both global and regional health check IDs. Added a new test case to verify regional health check functionality.

TAG=agy

CONV=6aff620c-e5a5-44eb-afe0-459cff820daa

tests/modules/net_lb_app_int/health-checks-regional.tfvars

@@ -0,0 +1,32 @@
+/**
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+backend_service_configs = {
+  default = {
+    backends = [{
+      group = "projects/myprj/zones/europe-west1-a/instanceGroups/my-ig"
+    }]
+    health_checks = ["regional"]
+  }
+}
+health_check_configs = {
+  regional = {
+    is_regional = true
+    tcp = {
+      port_specification = "USE_SERVING_PORT"
+    }
+  }
+}

tests/modules/net_lb_app_int/health-checks-regional.yaml

@@ -0,0 +1,20 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+counts:
+  google_compute_forwarding_rule: 1
+  google_compute_region_health_check: 1
+  google_compute_region_backend_service: 1
+  google_compute_region_target_http_proxy: 1
+  google_compute_region_url_map: 1

tests/modules/net_lb_app_int/tftest.yaml

@@ -25,6 +25,7 @@ tests:
   health-checks-http:
   health-checks-http2:
   health-checks-https:
+  health-checks-regional:
   health-checks-ssl:
   health-checks-tcp:
   https: