test: add serviceInterface test, fix fully-qualified import

- Add suspendFunctionsWithServiceInterface test covering service.mustache
- Replace java.nio.file.Files with already-imported Files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: thejeff77

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/spring/KotlinSpringServerCodegenTest.java

@@ -5133,8 +5133,32 @@ public void suspendFunctionsDefaultsToFalse() throws Exception {
         );
         // Verify no suspend keyword appears
         Path petApiPath = root.resolve("src/main/kotlin/org/openapitools/api/PetApi.kt");
-        String content = new String(java.nio.file.Files.readAllBytes(petApiPath));
+        String content = new String(Files.readAllBytes(petApiPath));
         Assert.assertFalse(content.contains("suspend fun"),
             "suspend should not be present when suspendFunctions is not enabled");
     }
+
+    @Test
+    public void suspendFunctionsWithServiceInterface() throws Exception {
+        Path root = generateApiSources(Map.of(
+                KotlinSpringServerCodegen.SUSPEND_FUNCTIONS, true,
+                KotlinSpringServerCodegen.SERVICE_INTERFACE, true,
+                KotlinSpringServerCodegen.DOCUMENTATION_PROVIDER, "none",
+                KotlinSpringServerCodegen.ANNOTATION_LIBRARY, "none",
+                KotlinSpringServerCodegen.USE_RESPONSE_ENTITY, true
+        ), Map.of(
+                CodegenConstants.MODELS, "false",
+                CodegenConstants.MODEL_TESTS, "false",
+                CodegenConstants.MODEL_DOCS, "false",
+                CodegenConstants.APIS, "true",
+                CodegenConstants.SUPPORTING_FILES, "false"
+        ));
+        verifyGeneratedFilesContain(
+                Map.of(
+                        root.resolve("src/main/kotlin/org/openapitools/api/PetApiService.kt"), List.of(
+                                "suspend fun deletePet(",
+                                "suspend fun getPetById(")
+                )
+        );
+    }
 }