azure-eventhub-checkpointstoretable needs typing updates for mypy version 1.14.1

**ACTION NEEDED:** This version of mypy will be merged on **2025-04-14**. The build will begin to fail for this library if errors are not fixed.

**Library name:** azure-eventhub-checkpointstoretable
**Mypy version:** 1.14.1
**Mypy errors:** [Link to build (2025-01-27)](https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4509986&view=logs&j=87f850b5-cf15-53c6-370a-4be21ee70ec7&t=59f5c573-b3ce-57f7-6a79-55fe2db3a175)
**How to fix:** Run the `next-mypy` tox command at the library package-level and resolve the typing errors.
1) `../azure-eventhub-checkpointstoretable>pip install "tox<5"`
2) `../azure-eventhub-checkpointstoretable>tox run -e next-mypy -c ../../../eng/tox/tox.ini --root .`

See the [Typing Guide](https://github.com/Azure/azure-sdk-for-python/blob/main/doc/dev/static_type_checking.md#run-mypy) for more information.