Add `mypy` presubmit to Python samples

[ohmayr]

It will be helpful to include a mypy presubmit to improve the end user experience and avoid type failures for users similar to: https://github.com/GoogleCloudPlatform/python-docs-samples/issues/13302.

Add a github presubmit that runs mypy on the samples to capture any type failures.

[Shriya-23]

Hi @ohmayr 👋

I’ve added a PR that implements the requested mypy presubmit workflow for Python samples: #13653

The workflow runs mypy on the samples/python directory for pull requests, helping catch type errors early and improve the developer experience.

Maintainers, happy to refine the workflow or PR if needed, and would appreciate your review/approval. Thank you!