fix: remove private _JSONTypeConverterUnhandled import from worker.py

This private symbol was used only as a return type annotation. In
some resolved versions of temporalio (within the >=1.18.2,<2 range),
this private name is not exported, causing an ImportError at worker
startup time in tutorial environments.

The sentinel value JSONTypeConverter.Unhandled (public API) is what
the method actually returns; the annotation is simplified to Any.

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

Author: scale-ballen

src/agentex/lib/core/temporal/workers/worker.py

@@ -24,7 +24,6 @@
     DefaultPayloadConverter,
     CompositePayloadConverter,
     JSONPlainPayloadConverter,
-    _JSONTypeConverterUnhandled,
 )
 from temporalio.contrib.openai_agents import OpenAIAgentsPlugin
 
@@ -45,7 +44,7 @@ def default(self, o: Any) -> Any:
 
 class DateTimeJSONTypeConverter(JSONTypeConverter):
     @override
-    def to_typed_value(self, hint: type, value: Any) -> Any | None | _JSONTypeConverterUnhandled:
+    def to_typed_value(self, hint: type, value: Any) -> Any:
         if hint == datetime.datetime:
             return datetime.datetime.fromisoformat(value)
         return JSONTypeConverter.Unhandled