Add test for invalid attach point

compiler-plugin-tests/src/test/java/io/ballerina/stdlib/nats/plugin/NatsCompilerPluginTest.java

@@ -541,6 +541,16 @@ public void testInvalidService30() {
         assertDiagnostic(diagnostic, CompilationErrors.NO_ANNOTATION);
     }
 
+    @Test(description = "NATS service with no service config or service name")
+    public void testInvalidService30() {
+        Package currentPackage = loadPackage("invalid_service_31");
+        PackageCompilation compilation = currentPackage.getCompilation();
+        DiagnosticResult diagnosticResult = compilation.diagnosticResult();
+        Assert.assertEquals(diagnosticResult.errors().size(), 1);
+        Diagnostic diagnostic = (Diagnostic) diagnosticResult.errors().toArray()[0];
+        assertDiagnostic(diagnostic, CompilationErrors.INVALID_SERVICE_ATTACH_POINT);
+    }
+
     private Package loadPackage(String path) {
         Path projectDirPath = RESOURCE_DIRECTORY.resolve(BALLERINA_SOURCES).resolve(path);
         BuildProject project = BuildProject.load(getEnvironmentBuilder(), projectDirPath);

compiler-plugin-tests/src/test/resources/ballerina_sources/invalid_service_31/Ballerina.toml

@@ -0,0 +1,4 @@
+[package]
+org = "nats_test"
+name = "invalid_service_31"
+version = "0.1.0"

compiler-plugin-tests/src/test/resources/ballerina_sources/invalid_service_31/service.bal

@@ -0,0 +1,28 @@
+// Copyright (c) 2023, WSO2 LLC. (https://www.wso2.com) All Rights Reserved.
+//
+// WSO2 LLC. licenses this file to you 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.
+
+import ballerinax/nats;
+
+listener nats:Listener subscription = new(nats:DEFAULT_URL);
+
+@display {
+    label: "natsService"
+}
+service nats:Service on subscription {
+
+    remote function onMessage(nats:Message message) returns error? {
+    }
+}