…crosoft#7153)

### Motivation and Context

We have mypy enabled on parts of the code base, but not all. The goal is
to enable it across the entire SK python code. As part of this, we've
broken up the work to tackle different sections. Additionally, we're
working to improve the code coverage for these sections of code.

<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->

### Description

This PR:
- turns on mypy for the openapi connector (plugin). Code adjustments
were made per mypy, with the exclusion of two places that contain a
`type: ignore` as the type was already valid, but mypy still complained.
- adds more unit test coverage for the OpenAPI models and classes. All
OpenAPI classes are now >95% covered.
- closes microsoft#7135 

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄