Fix mishandling of unions while freezing types in the mypy plugin

[dmontagu]

This addresses the specific issue brought up in #7399, but I am concerned that there is a bigger problem that I'm not adequately addressing here.

I will raise an issue in the mypy repository asking for more information, as I think the problem that the pydantic mypy plugin is hitting has something to do with a warning comment in code I adapted from the dataclasses plugin.

If we are going to merge this, we should update the mypy plugin tests to include the example from #7399 (including adding the unreachable warning).

Closes #7399

[dmontagu]

I think if this still passes CI let's just merge it @sydney-runkle, I created the mypy issue python/mypy#16087 but haven't heard anything back so I think it makes sense to address the case people are actually hitting and worry about doing a better job in the future if more issues get reported.

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[sydney-runkle]

Nice. Waiting on approval of the v2.5.0 release, then will merge this later today :).

And potentially waiting on @dmontagu adding the test mentioned in the description above.

[codspeed-hq]

CodSpeed Performance Report

Merging #7411 will not alter performance

_{Comparing dmontagu/mypy-plugin-weirdness (041715b) with main (7df25da)}

Summary

✅ 10 untouched benchmarks