refactor(reports): deduplicate database check and add precedence integration test

Extract repeated effective-database logic into a single computation
in UpdateReportScheduleCommand.validate(). Add integration test
asserting Report + nonexistent DB returns "not allowed" (not "does
not exist") to lock API contract precedence.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

superset/commands/report/update.py

@@ -100,23 +100,19 @@ class UpdateReportScheduleCommand(UpdateMixin, BaseReportScheduleCommand):
                     )
                 )
 
+        # Determine effective database state (payload overrides model)
+        if "database" in self._properties:
+            has_database = self._properties["database"] is not None
+        else:
+            has_database = self._model.database_id is not None
+
         # Validate database is not allowed on Report type
-        if report_type == ReportScheduleType.REPORT:
-            if "database" in self._properties:
-                has_database = self._properties["database"] is not None
-            else:
-                has_database = self._model.database_id is not None
-            if has_database:
-                exceptions.append(ReportScheduleDatabaseNotAllowedValidationError())
+        if report_type == ReportScheduleType.REPORT and has_database:
+            exceptions.append(ReportScheduleDatabaseNotAllowedValidationError())
 
         # Validate Alert has a database
-        if report_type == ReportScheduleType.ALERT:
-            if "database" in self._properties:
-                has_database = self._properties["database"] is not None
-            else:
-                has_database = self._model.database_id is not None
-            if not has_database:
-                exceptions.append(ReportScheduleAlertRequiredDatabaseValidationError())
+        if report_type == ReportScheduleType.ALERT and not has_database:
+            exceptions.append(ReportScheduleAlertRequiredDatabaseValidationError())
 
         # Validate if DB exists (for alerts)
         if report_type == ReportScheduleType.ALERT and database_id is not None:

tests/integration_tests/reports/api_tests.py

@@ -1654,6 +1654,38 @@ class TestReportSchedulesApi(SupersetTestCase):
             "message": {"database": "Database reference is not allowed on a report"}
         }
 
+    @pytest.mark.usefixtures("create_report_schedules")
+    def test_update_report_schedule_nonexistent_database_returns_not_allowed(self):
+        """
+        ReportSchedule API: Test Report + nonexistent DB returns 'not allowed',
+        not 'does not exist' — type invariant takes precedence.
+        """
+        self.login(ADMIN_USERNAME)
+
+        report_schedule = (
+            db.session.query(ReportSchedule)
+            .filter(ReportSchedule.name == "name1")
+            .one_or_none()
+        )
+        uri = f"api/v1/report/{report_schedule.id}"
+
+        # Transition to Report type first
+        rv = self.put_assert_metric(
+            uri,
+            {"type": ReportScheduleType.REPORT, "database": None},
+            "put",
+        )
+        assert rv.status_code == 200
+
+        # Report + nonexistent DB → 422 "not allowed" (not "does not exist")
+        database_max_id = db.session.query(func.max(Database.id)).scalar()
+        rv = self.put_assert_metric(uri, {"database": database_max_id + 1}, "put")
+        assert rv.status_code == 422
+        data = json.loads(rv.data.decode("utf-8"))
+        assert data == {
+            "message": {"database": "Database reference is not allowed on a report"}
+        }
+
     @pytest.mark.usefixtures("create_report_schedules")
     def test_update_alert_schedule_database_allowed(self):
         """