refactor: Import concrete BatchResult instead of protocol

Change __init__.py to import BatchResult from concurrency.models
(concrete implementation) rather than from types (protocol definition).
This provides users with the actual class implementation for better
IDE support and type checking.

The protocol remains in types.py for internal type checking, while
the public API exports the concrete dataclass implementation.

Author: yaythomas

src/aws_durable_execution_sdk_python/__init__.py

@@ -2,6 +2,8 @@
 
 # Main context - used in every durable function
 # Helper decorators - commonly used for step functions
+# Concurrency
+from aws_durable_execution_sdk_python.concurrency.models import BatchResult
 from aws_durable_execution_sdk_python.context import (
     DurableContext,
     durable_step,
@@ -20,7 +22,7 @@
 from aws_durable_execution_sdk_python.execution import durable_execution
 
 # Essential context types - passed to user functions
-from aws_durable_execution_sdk_python.types import BatchResult, StepContext
+from aws_durable_execution_sdk_python.types import StepContext
 
 __all__ = [
     "BatchResult",