[Bug]: fix the test cases that shut down event loop too early

[zhongkechen]

Expected Behavior

The futures need to be drained when the event loop is shut down in test cases

Actual Behavior

The tests are throwing the following warnings because the event loop is shut down too early.

Task was destroyed but it is pending!
task: <Task cancelling name='Task-356' coro=<Scheduler.call_later.<locals>.delayed_func() done, defined at /Users/frankchn/Library/Application Support/hatch/env/virtual/aws-durable-execution-sdk-python-monorepo/wf13lvoG/test/lib/python3.14/site-packages/aws_durable_execution_sdk_python_testing/scheduler.py:167> wait_for=<Future cancelled> cb=[_chain_future.<locals>._call_set_state() at /opt/homebrew/Cellar/python@3.14/3.14.5/Frameworks/Python.framework/Versions/3.14/lib/python3.14/asyncio/futures.py:400]>
Task was destroyed but it is pending!
task: <Task cancelling name='Task-338' coro=<Scheduler.call_later.<locals>.delayed_func() running at /Users/frankchn/Library/Application Support/hatch/env/virtual/aws-durable-execution-sdk-python-monorepo/wf13lvoG/test/lib/python3.14/site-packages/aws_durable_execution_sdk_python_testing/scheduler.py:170> wait_for=<Future cancelled> cb=[_chain_future.<locals>._call_set_state() at /opt/homebrew/Cellar/python@3.14/3.14.5/Frameworks/Python.framework/Versions/3.14/lib/python3.14/asyncio/futures.py:400]>
Task was destroyed but it is pending!
task: <Task cancelling name='Task-339' coro=<Scheduler.call_later.<locals>.delayed_func() running at /Users/frankchn/Library/Application Support/hatch/env/virtual/aws-durable-execution-sdk-python-monorepo/wf13lvoG/test/lib/python3.14/site-packages/aws_durable_execution_sdk_python_testing/scheduler.py:170> wait_for=<Future cancelled> cb=[_chain_future.<locals>._call_set_state() at /opt/homebrew/Cellar/python@3.14/3.14.5/Frameworks/Python.framework/Versions/3.14/lib/python3.14/asyncio/futures.py:400]>

Steps to Reproduce

N/A

SDK Version

N/A

Python Version

3.14

Is this a regression?

No

Last Working Version

No response

Additional Context

No response