Fix `pydantic` version tag

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As of #236, we have this version matrix for pydantic:

• Use version 2.9.2 for Python versions older than 3.14.
• Use the latest (as of this writing) version 2.12.5 for Python 3.14. This is the only pydantic version compatible with 3.14.

It is probably fine to use latest pydantic everywhere. I thought it would be better to enable testing for 3.14 and have at least one set of tests running latest pydantic than block until we can check against each of the changelists.

But... we should eventually streamline this to use one pydantic version requirement unless there is a reason not to.

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Solved by #350