Run trial tests on Python 3.14 in PRs

[anoadragon453]

This was prompted by #19132 being caught after the fact, rather than during review of #19055.

This adds two extra CI runs to each PR, with the benefit of catching places where we may introduce code that's incompatible with the latest Python version (but is compatible with the oldest).

To reduce the cost, we may want to only run the postgres Python 3.14 trial CI job on PRs, instead of both sqlite and postgres. This is because more tests are run on the postgres variant. Either the sqlite or postgres variant would have caught #19132.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[MadLittleMods]

👌 mint!