Clarify error message and add comment on builtin fallback

- Error message now says "one required positional argument" (functions
  with extra optional args are still valid)
- Added comment explaining why builtins skip signature validation

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

Author: CodeReclaimers

neat/aggregations.py

@@ -56,6 +56,8 @@ def validate_aggregation(function):
     try:
         signature = inspect.signature(function)
     except (TypeError, ValueError) as exc:
+        # CPython builtins (e.g. max, sum) often lack introspectable signatures.
+        # Skip signature validation for these; they are assumed correct.
         if isinstance(function, types.BuiltinFunctionType):
             return
         raise InvalidAggregationFunction("Unable to inspect aggregation callable signature.") from exc
@@ -64,7 +66,7 @@ def validate_aggregation(function):
         signature.bind(object())
     except TypeError as exc:
         raise InvalidAggregationFunction(
-            "A function taking a single positional argument is required"
+            "A callable with exactly one required positional argument is required"
         ) from exc