test(train-scheduling): cover the checkpoint backdating guard on the DTO

The IsNotBackdated validator was covered in isolation, but not on the DTO
that actually carries it. Asserts a backdated occurredAt is rejected, that
"now" passes, and that omitting the field still validates so the service can
stamp it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/edr-freight-api/src/modules/train-scheduling/dto/record-checkpoint.dto.spec.ts

@@ -0,0 +1,37 @@
+import { plainToInstance } from 'class-transformer';
+import { validate } from 'class-validator';
+
+import { RecordCheckpointDto } from './record-checkpoint.dto';
+
+const validateBody = (body: Record<string, unknown>) =>
+  validate(plainToInstance(RecordCheckpointDto, body));
+
+describe('RecordCheckpointDto', () => {
+  // The final checkpoint arrives the schedule, so a backdated one rewrites the
+  // journey after the fact. No UI sends occurredAt; the endpoint still accepts it.
+  it('rejects a backdated occurredAt', async () => {
+    const errors = await validateBody({
+      sequenceNo: 3,
+      occurredAt: new Date(Date.now() - 24 * 60 * 60 * 1000).toISOString(),
+    });
+
+    expect(errors).toHaveLength(1);
+    expect(errors[0].property).toBe('occurredAt');
+    expect(errors[0].constraints).toHaveProperty('IsNotBackdated');
+  });
+
+  it('accepts occurredAt of now', async () => {
+    const errors = await validateBody({
+      sequenceNo: 3,
+      occurredAt: new Date().toISOString(),
+    });
+
+    expect(errors).toHaveLength(0);
+  });
+
+  it('accepts a body that omits occurredAt, leaving the service to stamp it', async () => {
+    const errors = await validateBody({ sequenceNo: 0 });
+
+    expect(errors).toHaveLength(0);
+  });
+});