fix(code-mappings): Fix derive code mappings test for new unique constraint

Update test_post_existing_code_mapping to assert both mappings coexist
when stack_root matches but source_root differs, matching the new
(project, stack_root, source_root) unique constraint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: romtsn

tests/sentry/api/endpoints/issues/test_organization_derive_code_mappings.py

@@ -321,7 +321,12 @@ def test_post_existing_code_mapping(self) -> None:
         response = self.client.post(self.url, data=config_data, format="json")
         assert response.status_code == 201, response.content
 
-        new_code_mapping = RepositoryProjectPathConfig.objects.get(
+        # Both mappings should coexist: the original and the newly derived one
+        mappings = RepositoryProjectPathConfig.objects.filter(
             project=self.project, stack_root="/stack/root"
         )
-        assert new_code_mapping.source_root == "/source/root"
+        assert mappings.count() == 2
+        assert set(mappings.values_list("source_root", flat=True)) == {
+            "/source/root/wrong",
+            "/source/root",
+        }