Add exam start request and response DTOs

src/main/java/com/example/questionbank/web/rest/dto/ExamAttemptStartResult.java

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+package com.example.questionbank.web.rest.dto;
+
+import java.time.Instant;
+
+public record ExamAttemptStartResult(Long attemptId, Instant startedAt) {}

src/main/java/com/example/questionbank/web/rest/dto/StartExamRequest.java

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+package com.example.questionbank.web.rest.dto;
+
+import jakarta.validation.constraints.NotNull;
+
+public record StartExamRequest(@NotNull Long subjectId) {}