fix(api): declare UTF-8 plain-text ETL success responses

etl-service/src/main/java/com/xtrmetl/etl/controller/EtlController.java

@@ -17,6 +17,7 @@
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
 
+import java.nio.charset.StandardCharsets;
 import java.security.Principal;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -35,6 +36,12 @@ public class EtlController {
     /** Response header that tells a keyed client whether a prior success was replayed. */
     public static final String IDEMPOTENCY_REPLAYED_HEADER = "Idempotency-Replayed";
 
+    private static final MediaType UTF8_TEXT_PLAIN = new MediaType(
+            "text",
+            "plain",
+            StandardCharsets.UTF_8
+    );
+
     private final EtlService etlService;
     private final TargetConnectorDispatcher connectorDispatcher;
 
@@ -62,14 +69,14 @@ public EtlController(EtlService etlService, TargetConnectorDispatcher connectorD
      *
      * <p>The mapping does not constrain response negotiation to the successful representation.
      * This allows callers that accept only {@code application/problem+json} to receive typed
-     * failure responses before a successful text body is selected. Typed request and data-access
-     * failures retain their dedicated handlers; only an unexpected runtime failure raised during
-     * the service invocation is wrapped as an ETL internal failure.</p>
+     * failure responses before a successful UTF-8 text body is selected. Typed request and
+     * data-access failures retain their dedicated handlers; only an unexpected runtime failure
+     * raised during the service invocation is wrapped as an ETL internal failure.</p>
      *
      * @param jsonInput UTF-8 JSON array request body
      * @param idempotencyKey optional bounded client-generated retry key
      * @param principal authenticated caller namespace for keyed requests
-     * @return existing newline-delimited plain-text success response, plus replay metadata when keyed
+     * @return newline-delimited UTF-8 plain-text success response, plus replay metadata when keyed
      */
     @PostMapping("/process")
     @Observed(name = "etl.process", contextualName = "etl-processing")
@@ -104,7 +111,7 @@ public ResponseEntity<String> processData(
         }
 
         ResponseEntity.BodyBuilder response = ResponseEntity.ok()
-                .contentType(MediaType.TEXT_PLAIN);
+                .contentType(UTF8_TEXT_PLAIN);
         if (idempotencyKey != null) {
             response.header(IDEMPOTENCY_REPLAYED_HEADER, Boolean.toString(replayed));
         }
@@ -118,7 +125,7 @@ public ResponseEntity<String> processData(
      * direct controller tests and embedded integrations while delegating to the mapped method.</p>
      *
      * @param jsonInput UTF-8 JSON array request body
-     * @return existing newline-delimited plain-text success response
+     * @return newline-delimited UTF-8 plain-text success response
      */
     public ResponseEntity<String> processData(String jsonInput) {
         return processData(jsonInput, null, null);

etl-service/src/test/java/com/xtrmetl/etl/controller/EtlControllerUtf8ResponseTest.java

@@ -0,0 +1,45 @@
+package com.xtrmetl.etl.controller;
+
+import com.xtrmetl.etl.connector.ConnectorProperties;
+import com.xtrmetl.etl.connector.TargetConnectorDispatcher;
+import com.xtrmetl.etl.connector.TargetConnectorRegistry;
+import com.xtrmetl.etl.service.EtlService;
+import org.junit.jupiter.api.Test;
+import org.springframework.http.MediaType;
+import org.springframework.test.web.servlet.MockMvc;
+import org.springframework.test.web.servlet.setup.MockMvcBuilders;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.header;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+class EtlControllerUtf8ResponseTest {
+
+    @Test
+    void successfulUnicodeResultDeclaresUtf8TextRepresentation() throws Exception {
+        EtlService etlService = mock(EtlService.class);
+        TargetConnectorDispatcher dispatcher = new TargetConnectorDispatcher(
+                new TargetConnectorRegistry(),
+                new ConnectorProperties()
+        );
+        MockMvc mockMvc = MockMvcBuilders.standaloneSetup(
+                        new EtlController(etlService, dispatcher)
+                )
+                .setControllerAdvice(new EtlApiProblemHandler())
+                .build();
+
+        String request = "[{\"id\":\"레코드_α\"}]";
+        String response = "Processed: 레코드_α";
+        when(etlService.processData(request)).thenReturn(response);
+
+        mockMvc.perform(post("/api/etl/process")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(request))
+                .andExpect(status().isOk())
+                .andExpect(header().string("Content-Type", "text/plain;charset=UTF-8"))
+                .andExpect(content().string(response));
+    }
+}