Refactor PlansRepository to optimize data loading and enhance error handling

- Replaced the previous hydration logic with a new method, `loadChildrenForRows`, to efficiently load related data for plans in a single query.
- Implemented a `groupByPlanId` utility to organize related entities by plan ID, improving data retrieval performance.
- Updated error handling in the `getPlan` method to throw a `NotFoundException` if a plan is not found.
- Modified end-to-end tests to validate the new structure of the response, ensuring that related destinations are correctly included in the API output.

test/plans.e2e-spec.ts

@@ -251,7 +251,17 @@ describe('Plans (e2e)', () => {
       .query({ employeeId, purpose: 'sales', date: from })
       .set('Authorization', `Bearer ${adminAccessToken}`)
       .expect(200);
-    const planId = (list.body.data as Array<{ id: string }>)[0].id;
+    const listed = (
+      list.body as {
+        data: Array<{
+          id: string;
+          destinations: Array<{ customer: { id: string } | null }>;
+        }>;
+      }
+    ).data;
+    expect(listed[0].destinations).toHaveLength(1);
+    expect(listed[0].destinations[0].customer?.id).toBe(customerId);
+    const planId = listed[0].id;
 
     const added = await request(app.getHttpServer())
       .post(`/plans/${planId}/destinations`)
@@ -259,13 +269,16 @@ describe('Plans (e2e)', () => {
       .send({ customerId: extraCustomerId })
       .expect(201);
     expect(
-      (added.body as { destinations: Array<{ customerId: string }> })
-        .destinations,
+      (added.body as { destinations: unknown[] }).destinations,
     ).toHaveLength(2);
 
     const extraDest = (
-      added.body as { destinations: Array<{ id: string; customerId: string }> }
-    ).destinations.find((d) => d.customerId === extraCustomerId);
+      added.body as {
+        destinations: Array<{ id: string; customer: { id: string } | null }>;
+      }
+    ).destinations.find(
+      (destination) => destination.customer?.id === extraCustomerId,
+    );
     const removed = await request(app.getHttpServer())
       .delete(`/plans/${planId}/destinations/${extraDest?.id}`)
       .set('Authorization', `Bearer ${adminAccessToken}`)