azure-developer-loadtesting needs typing updates for mypy version 1.14.1

[azure-sdk]

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-developer-loadtesting
Mypy version: 1.14.1
Mypy Build: Link to build (2025-04-06)
How to fix: Run the next-mypy tox command at the library package-level and resolve the typing errors.

1. ../azure-developer-loadtesting>pip install "tox<5"
2. ../azure-developer-loadtesting>tox run -e next-mypy -c ../../../eng/tox/tox.ini --root .

See the Typing Guide for more information.