fix(internal): add back custom header naming support (#861)

Author: stainless-app[bot]

src/anthropic/_models.py

@@ -442,10 +442,16 @@ def construct_type(*, value: object, type_: object) -> object:
 
     If the given value does not match the expected type then it is returned as-is.
     """
+
+    # store a reference to the original type we were given before we extract any inner
+    # types so that we can properly resolve forward references in `TypeAliasType` annotations
+    original_type = None
+
     # we allow `object` as the input type because otherwise, passing things like
     # `Literal['value']` will be reported as a type error by type checkers
     type_ = cast("type[object]", type_)
     if is_type_alias_type(type_):
+        original_type = type_  # type: ignore[unreachable]
         type_ = type_.__value__  # type: ignore[unreachable]
 
     # unwrap `Annotated[T, ...]` -> `T`
@@ -462,7 +468,7 @@ def construct_type(*, value: object, type_: object) -> object:
 
     if is_union(origin):
         try:
-            return validate_type(type_=cast("type[object]", type_), value=value)
+            return validate_type(type_=cast("type[object]", original_type or type_), value=value)
         except Exception:
             pass