feat: Add ExecutionContext with durable execution ARN

Add immutable ExecutionContext dataclass to provide readonly access to
execution-level metadata. The context contains the durable execution ARN
which uniquely identifies the execution instance within AWS.

Changes:
- Create ExecutionContext frozen dataclass with durable_execution_arn field
- Add execution_context parameter to DurableContext constructor
- Update from_lambda_context() to create ExecutionContext from state
- Update create_child_context() to propagate execution_context
- Export ExecutionContext in public API
- Add create_test_context() helper in test files for easier instantiation
- Update all test files to use new helper function

The ExecutionContext is automatically created in from_lambda_context()
using the state's durable_execution_arn, keeping the API simple while
maintaining immutability and thread safety.

closes #283

Author: yaythomas

src/aws_durable_execution_sdk_python/context.py

@@ -2,6 +2,7 @@
 
 import hashlib
 import logging
+from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any, Concatenate, Generic, ParamSpec, TypeVar
 
 from aws_durable_execution_sdk_python.config import (
@@ -74,6 +75,20 @@
 PASS_THROUGH_SERDES: SerDes[Any] = PassThroughSerDes()
 
 
+@dataclass(frozen=True)
+class ExecutionContext:
+    """Readonly metadata about the current durable execution context.
+
+    This class provides immutable access to execution-level metadata.
+
+    Attributes:
+        durable_execution_arn: The Amazon Resource Name (ARN) of the current
+            durable execution.
+    """
+
+    durable_execution_arn: str
+
+
 def durable_step(
     func: Callable[Concatenate[StepContext, Params], T],
 ) -> Callable[Params, Callable[[StepContext], T]]:
@@ -218,11 +233,13 @@ class DurableContext(DurableContextProtocol):
     def __init__(
         self,
         state: ExecutionState,
+        execution_context: ExecutionContext,
         lambda_context: LambdaContext | None = None,
         parent_id: str | None = None,
         logger: Logger | None = None,
     ) -> None:
         self.state: ExecutionState = state
+        self.execution_context: ExecutionContext = execution_context
         self.lambda_context = lambda_context
         self._parent_id: str | None = parent_id
         self._step_counter: OrderedCounter = OrderedCounter()
@@ -245,6 +262,9 @@ def from_lambda_context(
     ):
         return DurableContext(
             state=state,
+            execution_context=ExecutionContext(
+                durable_execution_arn=state.durable_execution_arn
+            ),
             lambda_context=lambda_context,
             parent_id=None,
         )
@@ -254,6 +274,7 @@ def create_child_context(self, parent_id: str) -> DurableContext:
         logger.debug("Creating child context for parent %s", parent_id)
         return DurableContext(
             state=self.state,
+            execution_context=self.execution_context,
             lambda_context=self.lambda_context,
             parent_id=parent_id,
             logger=self.logger.with_log_info(