fix: handle types.UnionType in from_dict_deep, fix missed List reference

On Python 3.10-3.12, the `X | Y` syntax creates a `types.UnionType`
which is distinct from `typing.Union`. The `from_dict_deep` method
in `SerializableDataClass` only checked for `typing.Union` via
`get_origin`, so it failed to deserialize union-typed fields when
the annotation used pipe syntax (e.g. `PromptBlockData`).

Also fixes a missed `cast(List, ...)` → `cast(list, ...)` in
langchain test_callbacks.py that caused a pylint E0602.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Nova (SFK)

py/src/braintrust/integrations/langchain/test_callbacks.py

@@ -399,7 +399,7 @@ def test_parallel_execution(logger_memory_logger):
 
     map_chain.invoke({"topic": "bear"}, config={"callbacks": [cast(BaseCallbackHandler, handler)]})
 
-    spans = cast(List, memory_logger.pop())
+    spans = cast(list, memory_logger.pop())
 
     # Find the LLM spans
     llm_spans = find_spans_by_attributes(spans, name="ChatOpenAI")

py/src/braintrust/serializable_data_class.py

@@ -1,5 +1,6 @@
 import dataclasses
 import json
+import types
 from typing import Union, get_origin
 
 
@@ -39,7 +40,7 @@ def from_dict_deep(cls, d: dict):
                 and issubclass(fields[k].type, SerializableDataClass)
             ):
                 filtered[k] = fields[k].type.from_dict_deep(v)
-            elif get_origin(fields[k].type) == Union:
+            elif get_origin(fields[k].type) is Union or isinstance(fields[k].type, types.UnionType):
                 for t in fields[k].type.__args__:
                     if t == type(None) and v is None:
                         filtered[k] = None