(BSR)[API] fix: flush the deposit manually after object creation

A database object should be manually flushed if the business logic
depends on a field that is populated by the server, like the creation
date. Furthermore, relying on autoflush is a bad practice because some
routes disable it in order to have more control on the SQLAlchemy session.

api/src/pcapi/core/finance/api.py

@@ -2605,6 +2605,7 @@ def create_deposit(
         expirationDate=granted_deposit.expiration_date,
     )
     db.session.add(deposit)
+    db.session.flush()
 
     # Edge-cases: Validation of the registration occurred over a birthday
     # Then we need to add recredit to compensate

api/tests/core/finance/test_api.py

@@ -3621,6 +3621,27 @@ def test_create_underage_deposit_and_recredit_sum_of_15_17(self, age, expected_a
         assert deposit.amount == expected_amount
         assert deposit.user.id == beneficiary.id
 
+    def test_create_underage_deposit_with_birthday_since_registration(self):
+        sixteen_years_ago = datetime.datetime.utcnow() - relativedelta(years=16)
+        beneficiary = users_factories.UserFactory(validatedBirthDate=sixteen_years_ago.date())
+        fraud_factories.BeneficiaryFraudCheckFactory(
+            user=beneficiary,
+            status=fraud_models.FraudCheckStatus.OK,
+            type=fraud_models.FraudCheckType.EDUCONNECT,
+            eligibilityType=users_models.EligibilityType.UNDERAGE,
+            resultContent=fraud_factories.EduconnectContentFactory(
+                registration_datetime=datetime.datetime.utcnow() - relativedelta(days=1)
+            ),
+        )
+
+        with db.session.no_autoflush:
+            deposit = api.create_deposit(
+                beneficiary, "created by test", beneficiary.eligibility, age_at_registration=15
+            )
+
+        assert deposit.type == models.DepositType.GRANT_15_17
+        assert deposit.amount == Decimal(20 + 30)
+
     @time_machine.travel("2022-09-05 09:00:00")
     def test_create_18_years_old_deposit(self):
         beneficiary = users_factories.UserFactory()