fix: use StandardCharsets.UTF_8 to avoid forbidden API violation

The forbiddenapis plugin rejects String(byte[]) without explicit charset.

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

@@ -5133,7 +5133,7 @@ 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(Files.readAllBytes(petApiPath));
+        String content = new String(Files.readAllBytes(petApiPath), java.nio.charset.StandardCharsets.UTF_8);
         Assert.assertFalse(content.contains("suspend fun"),
             "suspend should not be present when suspendFunctions is not enabled");
     }