feat: export __version__ from package root

- Re-export __version__ from __about__ module at package level
- Add __version__ to __all__ for explicit public API declaration
- Add test to verify __version__ is accessible

Enables the standard Python version access pattern:

    import aws_durable_execution_sdk_python
    print(aws_durable_execution_sdk_python.__version__)

This follows the convention used by boto3, botocore, and AWS Lambda
Powertools, allowing users to access version information without
knowledge of internal module structure.

Author: Iamrodos

src/aws_durable_execution_sdk_python/__init__.py

@@ -1,5 +1,8 @@
 """AWS Lambda Durable Executions Python SDK."""
 
+# Package metadata
+from aws_durable_execution_sdk_python.__about__ import __version__
+
 # Main context - used in every durable function
 # Helper decorators - commonly used for step functions
 # Concurrency
@@ -31,6 +34,7 @@
     "InvocationError",
     "StepContext",
     "ValidationError",
+    "__version__",
     "durable_execution",
     "durable_step",
     "durable_wait_for_callback",

tests/durable_executions_python_language_sdk_test.py

@@ -4,3 +4,12 @@
 def test_aws_durable_execution_sdk_python_importable():
     """Test aws_durable_execution_sdk_python is importable."""
     import aws_durable_execution_sdk_python  # noqa: PLC0415, F401
+
+
+def test_version_is_accessible():
+    """Test __version__ is accessible from package root."""
+    import aws_durable_execution_sdk_python  # noqa: PLC0415
+
+    assert hasattr(aws_durable_execution_sdk_python, "__version__")
+    assert isinstance(aws_durable_execution_sdk_python.__version__, str)
+    assert len(aws_durable_execution_sdk_python.__version__) > 0