Disable explicit mypy tests?

What is your issue?

The explicit mypy tests introduced in https://github.com/pydata/xarray/pull/10038 seem to be quite fragile, e.g., they are currently broken at head and I know have been broken at head previously at least once. In neither case was there a real regression.

I see at least three issues with them:

1. They seem to be particularly prone to version skew issues where they fail on GitHub but pass locally due to slight differences in environments. I think the current failure is due to mis-matched NumPy versions. (This might be improved when we switch to pixi.)
2. They are really slow. Testing ~40 specific errors takes ~9 minutes on a single GitHub CI worker, which is not a great use of CI resources and clearly not scalable. Currently we use two CI workers just to test typing for the pipe method!
3. They are inherently very fragile, checking for exact messages printed by mypy. Pretty much any package update causes error messages to change, which means these tests break, whether or not there is a real underlying issue.

Start by locating the explicit mypy tests introduced in PR #10038 and the GitHub CI jobs that run them. Review their current failures, runtime, and typing coverage; done means the fragile explicit tests are disabled without obscuring the remaining typing checks.