fix(bundler): reject non-list 'catalogs' in bundle-catalogs.yml with a clean error

_merge_config guarded only 'if not catalogs: return', so a non-empty scalar
(catalogs: 5) passed through and raised a raw 'TypeError: int object is not
iterable' from the loop below — escaping the module's BundlerError error
contract. The sibling reader of the same file (commands_impl/catalog_config.py,
used by 'bundle catalog list') already raises an actionable BundlerError for the
identical mis-shape. Add the same isinstance(list) guard so both readers of
bundle-catalogs.yml agree.

Test: 'catalogs: 5' raises BundlerError('...must be a list...') (fails before:
raw TypeError).

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

Author: jawwad-ali

src/specify_cli/bundler/models/catalog.py

@@ -253,6 +253,16 @@ def _merge_config(by_id: dict[str, CatalogSource], config_path: Path, scope: Sco
     catalogs = data.get("catalogs") if isinstance(data, dict) else None
     if not catalogs:
         return
+    if not isinstance(catalogs, list):
+        # A non-empty scalar (``catalogs: 5``) passes the falsy check above and
+        # would raise a raw ``TypeError: 'int' object is not iterable`` from the
+        # loop below. Report the same actionable BundlerError the sibling reader
+        # of this file raises (commands_impl/catalog_config.py) so both readers
+        # of bundle-catalogs.yml agree.
+        raise BundlerError(
+            f"Malformed catalog config at {config_path}: 'catalogs' must be a "
+            f"list, got {type(catalogs).__name__}."
+        )
     for raw in catalogs:
         src = CatalogSource.from_dict(raw, scope)
         by_id[src.id] = src

tests/contract/test_catalog_schema.py

@@ -43,6 +43,18 @@ def test_builtin_default_stack_when_no_config(tmp_path: Path):
     assert all(s.scope is Scope.BUILTIN for s in sources)
 
 
+def test_non_list_catalogs_raises_actionable_error(tmp_path: Path):
+    """A scalar ``catalogs:`` value raises a clean BundlerError, not a raw
+    'int object is not iterable' TypeError — matching what the sibling reader
+    (bundle catalog list) already reports for the same file."""
+    make_project(tmp_path)
+    (tmp_path / ".specify" / "bundle-catalogs.yml").write_text(
+        "catalogs: 5\n", encoding="utf-8"
+    )
+    with pytest.raises(BundlerError, match="must be a list"):
+        load_source_stack(tmp_path)
+
+
 def test_project_config_overrides_same_id(tmp_path: Path):
     make_project(tmp_path)
     config = {