Implement better unit testing for generate mapping

[joanise]

The recent upgrade to Pydantic 2 broke g2p generate-mapping, but we don't have adequate unit testing and the bugs introduced were not caught. See #293 fixing that.

So we should implement more comprehensive tests for generate-mapping, including actually creating a generated mapping in CI and validating it, somehow. test_z_ is probably where that will have to go, since this test will modify the in-memory database contents.