artifact-registry: support common_repository in maven, npm, and python remote formats (#3944)

Add `common_repository` support to `maven`, `npm`, and `python` remote repository configurations in the `artifact-registry` module. This replaces the deprecated `custom_repository` feature which is now discouraged by the provider.

Existing README example `registry-mirror` has been updated to use `common_repository`. A legacy test case `legacy_custom_repo` has been added to the bottom of `README.md` to ensure backward compatibility for `custom_repository` continues to work.

TAG=agy
CONV=ffe77e65-ccef-4701-95e6-4ba2d2446f1b

tests/modules/artifact_registry/examples/legacy-custom.yaml

@@ -0,0 +1,57 @@
+# 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
+#
+#     https://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.
+
+values:
+  module.legacy_custom_repo.google_artifact_registry_repository.registry:
+    cleanup_policies: []
+    cleanup_policy_dry_run: null
+    description: Terraform-managed registry
+    docker_config: []
+    effective_labels:
+      goog-terraform-provisioned: 'true'
+    format: MAVEN
+    kms_key_name: null
+    labels: null
+    location: europe-west1
+    maven_config: []
+    mode: REMOTE_REPOSITORY
+    project: myproject
+    remote_repository_config:
+    - apt_repository: []
+      common_repository: []
+      description: null
+      disable_upstream_validation: null
+      docker_repository: []
+      maven_repository:
+      - custom_repository:
+        - uri: https://example.com
+        public_repository: null
+      npm_repository: []
+      python_repository: []
+      upstream_credentials: []
+      yum_repository: []
+    repository_id: legacy-custom
+    terraform_labels:
+      goog-terraform-provisioned: 'true'
+    timeouts: null
+    virtual_repository_config: []
+    vulnerability_scanning_config:
+    - enablement_config: null
+
+counts:
+  google_artifact_registry_repository: 1
+  modules: 1
+  resources: 1
+
+outputs: {}

tests/modules/artifact_registry/examples/remote-credentials.yaml

@@ -4,7 +4,7 @@
 # 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
+#     https://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,
@@ -58,13 +58,11 @@ values:
     project: test-ar
     remote_repository_config:
     - apt_repository: []
-      common_repository: []
+      common_repository:
+      - uri: https://example.com
       description: null
       disable_upstream_validation: null
-      docker_repository:
-      - custom_repository:
-        - uri: https://example.com
-        public_repository: null
+      docker_repository: []
       maven_repository: []
       npm_repository: []
       python_repository: []

tests/modules/artifact_registry/remote_maven_common.tfvars

@@ -0,0 +1,26 @@
+/**
+ * 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.
+ */
+
+project_id = "my-project"
+location   = "europe-west1"
+name       = "maven-remote"
+format = {
+  maven = {
+    remote = {
+      common_repository = "https://repo1.maven.org/maven2/"
+    }
+  }
+}

tests/modules/artifact_registry/remote_maven_common.yaml

@@ -0,0 +1,24 @@
+# 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.
+
+values:
+  google_artifact_registry_repository.registry:
+    format: MAVEN
+    mode: REMOTE_REPOSITORY
+    location: europe-west1
+    project: my-project
+    repository_id: maven-remote
+    remote_repository_config:
+    - common_repository:
+      - uri: https://repo1.maven.org/maven2/

tests/modules/artifact_registry/tftest.yaml

@@ -17,3 +17,4 @@ module: modules/artifact-registry
 tests:
   context:
   universe:
+  remote_maven_common: