fix: do not override Document._class_id if it was explicitly set on the document class

[volfpeter]

Fixes #1132

[staticxterm]

Hi @volfpeter. Thank you for the issue report/PR.
Please also write tests for the scenarios that you described in the issue report, to have failing tests that test and reproduce this behavior prior to the changes made here in this PR.

[volfpeter]

Hi, Thanks for the quick response. I just fixed the existing tests. I'll add a new test for this specific case.

[volfpeter]

@staticxterm I added basically the same set of tests that currently exist for the standard inheritance case. Admittedly, parametrized tests would have been nicer, but I didn't want to start that refactoring, because I have very limited time (and kind of need this fix as soon as possible). Let me know if I should change anything.

[volfpeter]

@staticxterm Do you have an idea why the "3.13, 8.0, 1.10.18" test failed? I ran the test suite locally with the same Python and package versions (Python 3.13, pytest 8.3.4, and both pydantic 1.10.18 and 1.10.21), and all tests passed.

[staticxterm]

Hi @volfpeter, the test failed because yes there is some flakiness. I'll look into it.
I'll rerun the failed test.

[volfpeter]

Thanks! First I thought this was the case, but then it seemed that locally it's always passing, and the CI job is always failing, and I didn't want to trigger to useless reruns.