Tests: Cover schema changes and appends during commit retry

[mifuha]

Rationale for this change

Add a test for a transaction that adds a nullable column and appends a row, while another writer commits first.

The test checks that the first writer retries and keeps the new column, its field ID, and all three rows. It also checks the snapshot's parent and schema ID, and that reloading the table gives the same result.

Are these changes tested?

Yes. The new test passes for all three existing catalog fixtures. make test passes with 4,239 passed, 5 skipped, and 126 deselected on Python 3.12. make lint also passes.

Are there any user-facing changes?

No. This adds test coverage only.

AI-assisted implementation.

[mifuha]

Hi @kevinjqliu, just following up on this PR.

I saw that #4025 fixed the setup-uv allowlist issue, but this PR still shows the earlier CI startup failures. Should I update my branch from main to pick up the fix?
This PR only adds regression coverage, and the tests and lint checks passed locally. I’d appreciate a review when convenient, or a pointer to the right reviewer.

Thanks!