feat: 新增 GET /exchange/attempts/latest，区分实验"已完成"与"从未开始"

attempts/current 只查 IN_PROGRESS/SUBMITTED，实验走完后返回 404，
前端无法区分"已完成"和"从未开始"，会静默新建一个空实验，导致页面
回显旧进度而后端动作历史为空。

新增 latest 接口返回最近一次未取消的实验（含 COMPLETED），已取消的不返回。
current 的行为不变。

src/main/java/com/yau/digitalrmb/exchange/interfaces/rest/ExchangeTrainingAttemptController.java

@@ -54,6 +54,13 @@ public class ExchangeTrainingAttemptController {
         return ok(attempts.current(ExperimentModule.EXCHANGE, subject(users.getCurrentUser())));
     }
 
+    @GetMapping("/latest")
+    @Operation(summary = "查询最近一次兑换实验",
+            description = "含已完成的实验，用于区分「从未开始」与「已完成」；已取消的不返回。")
+    public ApiResponse<ExperimentAttemptView> latest() {
+        return ok(attempts.latest(ExperimentModule.EXCHANGE, subject(users.getCurrentUser())));
+    }
+
     @GetMapping("/{attemptId}")
     @Operation(summary = "查询兑换实验详情和已保存动作")
     public ApiResponse<ExperimentAttemptView> detail(@PathVariable UUID attemptId) {

src/main/java/com/yau/digitalrmb/training/attempt/application/ExperimentAttemptService.java

@@ -50,6 +50,13 @@ public class ExperimentAttemptService {
         return toAttemptView(attempt, repository.findActions(attempt.getId()));
     }
 
+    @Transactional(readOnly = true)
+    public ExperimentAttemptView latest(ExperimentModule module, ExperimentSubject subject) {
+        ExperimentAttempt attempt = repository.findLatest(module, subject)
+                .orElseThrow(() -> notFound("未找到兑换实验记录"));
+        return toAttemptView(attempt, repository.findActions(attempt.getId()));
+    }
+
     @Transactional(readOnly = true)
     public ExperimentAttemptView detail(UUID attemptId, ExperimentSubject subject) {
         ExperimentAttempt attempt = repository.findById(attemptId, subject)

src/main/java/com/yau/digitalrmb/training/attempt/domain/ExperimentAttemptRepository.java

@@ -7,6 +7,8 @@ import java.util.UUID;
 public interface ExperimentAttemptRepository {
     ExperimentAttempt create(ExperimentModule module, ExperimentSubject subject);
     Optional<ExperimentAttempt> findCurrent(ExperimentModule module, ExperimentSubject subject);
+    /** 最近一次未取消的实验，含已完成的；用于区分「从未开始」与「已完成」。 */
+    Optional<ExperimentAttempt> findLatest(ExperimentModule module, ExperimentSubject subject);
     Optional<ExperimentAttempt> findById(UUID id, ExperimentSubject subject);
     Optional<ExperimentAttempt> findByIdForUpdate(UUID id, ExperimentSubject subject);
     void save(ExperimentAttempt attempt);

src/main/java/com/yau/digitalrmb/training/attempt/infrastructure/JdbcExperimentAttemptRepository.java

@@ -59,6 +59,17 @@ public class JdbcExperimentAttemptRepository implements ExperimentAttemptReposit
         return optional(values);
     }
 
+    @Override
+    public Optional<ExperimentAttempt> findLatest(ExperimentModule module, ExperimentSubject subject) {
+        List<ExperimentAttempt> values = jdbc.query("SELECT * FROM training_experiment_attempt WHERE module_code=? "
+                        + "AND user_id=? AND school_id=? AND class_id=? "
+                        + "AND organization_id=? "
+                        + "AND status IN ('IN_PROGRESS','SUBMITTED','COMPLETED') ORDER BY attempt_no DESC LIMIT 1",
+                attemptMapper(), module.name(), subject.getUserId(), subject.getSchoolId(), subject.getClassId(),
+                subject.getOrganizationId());
+        return optional(values);
+    }
+
     @Override
     public Optional<ExperimentAttempt> findById(UUID id, ExperimentSubject subject) {
         return findById(id, subject, false);

src/test/java/com/yau/digitalrmb/exchange/interfaces/rest/ExchangeTrainingAttemptControllerTest.java

@@ -87,6 +87,8 @@ class ExchangeTrainingAttemptControllerTest {
 
         mockMvc.perform(get("/api/v1/exchange/attempts/current").with(jwtForUser()))
                 .andExpect(status().isOk()).andExpect(jsonPath("$.data.attemptId").value(attemptId.toString()));
+        mockMvc.perform(get("/api/v1/exchange/attempts/latest").with(jwtForUser()))
+                .andExpect(status().isOk()).andExpect(jsonPath("$.data.attemptId").value(attemptId.toString()));
         mockMvc.perform(get("/api/v1/exchange/attempts/{id}", attemptId).with(jwtForUser()))
                 .andExpect(status().isOk()).andExpect(jsonPath("$.data.attemptId").value(attemptId.toString()));
         mockMvc.perform(post("/api/v1/exchange/attempts/{id}/cancel", attemptId).with(jwtForUser()))

src/test/java/com/yau/digitalrmb/training/attempt/infrastructure/JdbcExperimentAttemptRepositoryTest.java

@@ -41,6 +41,34 @@ class JdbcExperimentAttemptRepositoryTest {
         assertEquals(second.getId(), repository.findCurrent(ExperimentModule.EXCHANGE, subject).get().getId());
     }
 
+    @Test
+    void findLatestReturnsCompletedAttemptButCurrentDoesNot() {
+        ExperimentSubject subject = new ExperimentSubject("user-latest", 10L, 20L, "ORG-1");
+
+        ExperimentAttempt first = repository.create(ExperimentModule.EXCHANGE, subject);
+        first.complete(java.time.Instant.now());
+        repository.save(first);
+
+        assertFalse(repository.findCurrent(ExperimentModule.EXCHANGE, subject).isPresent());
+        assertEquals(first.getId(), repository.findLatest(ExperimentModule.EXCHANGE, subject).get().getId());
+    }
+
+    @Test
+    void findLatestSkipsCancelledAttemptsAndPrefersTheNewestOne() {
+        ExperimentSubject subject = new ExperimentSubject("user-latest-2", 10L, 20L, "ORG-1");
+
+        ExperimentAttempt first = repository.create(ExperimentModule.EXCHANGE, subject);
+        first.complete(java.time.Instant.now());
+        repository.save(first);
+        ExperimentAttempt second = repository.create(ExperimentModule.EXCHANGE, subject);
+        assertEquals(second.getId(), repository.findLatest(ExperimentModule.EXCHANGE, subject).get().getId());
+
+        second.cancel();
+        repository.save(second);
+        assertEquals(first.getId(), repository.findLatest(ExperimentModule.EXCHANGE, subject).get().getId());
+        assertFalse(repository.findLatest(ExperimentModule.PAYMENT, subject).isPresent());
+    }
+
     @Test
     void scopesAttemptReadsToUserSchoolClassAndOrganization() {
         ExperimentSubject owner = new ExperimentSubject("user-1", 10L, 20L, "ORG-1");