fix(bundler): reject a top-level non-mapping bundle-catalogs.yml in _merge_config

_merge_config silently ignored a top-level non-mapping document (a YAML list
or scalar) — `data.get("catalogs") if isinstance(data, dict) else None` made
it fall through to the built-in default stack — while the sibling reader of
the SAME file (commands_impl/catalog_config._read) raises "expected a mapping
at the top level". #3623 already made the inner non-list `catalogs` value
agree between the two readers; this closes the remaining top-level-shape gap
so both readers reject the same malformed documents.

An empty file (load_yaml coerces to {}), absent `catalogs`, and `catalogs: []`
all remain no-ops.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jawwad-ali

src/specify_cli/bundler/models/catalog.py

@@ -250,7 +250,17 @@ def _merge_config(by_id: dict[str, CatalogSource], config_path: Path, scope: Sco
     if not config_path.exists():
         return
     data = load_yaml(config_path)
-    catalogs = data.get("catalogs") if isinstance(data, dict) else None
+    if not isinstance(data, dict):
+        # A top-level non-mapping (a YAML list or scalar) is malformed. The
+        # sibling reader of the SAME file (commands_impl/catalog_config._read)
+        # raises here; #3623 already made the inner non-list `catalogs` value
+        # agree between the two readers, and this closes the remaining
+        # top-level-shape gap so both readers reject the same documents.
+        raise BundlerError(
+            f"Malformed catalog config at {config_path}: expected a mapping at "
+            f"the top level, got {type(data).__name__}."
+        )
+    catalogs = data.get("catalogs")
     if catalogs is None:
         return
     if not isinstance(catalogs, list):

tests/contract/test_catalog_schema.py

@@ -68,6 +68,17 @@ def test_falsy_non_list_catalogs_still_raises(tmp_path: Path, value: str):
         load_source_stack(tmp_path)
 
 
+@pytest.mark.parametrize("body", ["- a\n- b\n", "42\n"])
+def test_toplevel_non_mapping_raises(tmp_path: Path, body: str):
+    """A top-level non-mapping bundle-catalogs.yml (a YAML list or scalar) must
+    raise, matching the sibling reader (catalog_config._read) — not silently
+    fall back to the built-in default stack."""
+    make_project(tmp_path)
+    (tmp_path / ".specify" / "bundle-catalogs.yml").write_text(body, encoding="utf-8")
+    with pytest.raises(BundlerError, match="expected a mapping at the top level"):
+        load_source_stack(tmp_path)
+
+
 @pytest.mark.parametrize("body", ["catalogs:\n", "catalogs: []\n"])
 def test_absent_or_empty_catalogs_is_noop(tmp_path: Path, body: str):
     """An absent (``None``) or empty-list ``catalogs:`` is valid: it contributes