Assert the 2023-24 Class 4 lower profits limit instead of expecting it to fail

[MaxGhenis]

Refs #1968 (closed by #2058)

What and why

test_lower_thresholds_align_with_the_personal_allowance[2023] carried a non-strict xfail for #1968: the model's 2023-24 Class 4 lower profits limit was £11,908, not the £12,570 that NICA 2022 s2 sets from 6 April 2023. #2058 fixed that on 10 October (class_4/thresholds/lower_profits_limit.yaml has 12,570 from 2023-04-06), so the case has been XPASSing. A non-strict xfail that passes never fails, and if the LPL went back to £11,908 the case would show as an expected failure, not a red test. So the test protected nothing for 2023-24.

This removes the marker, so 2023 is an ordinary case of the alignment test, and drops the #1968 exception from the invariant lists of this module and of test_ni_threshold_freeze.py, which repeated it.

Part of workstream W2 of the Autumn Budget CI plan: the follow-up CI change fails any skip or expected failure that isn't on a reviewed allowlist, and this marker would otherwise have needed an entry.

Review

An independent Opus 5.5 check confirmed the fix on main (the YAML value and its NICA 2022 s2 reference, and the 30 April fiscal-year sampling) and found the repeated #1968 sentence in test_ni_threshold_freeze.py, now removed.

Test plan

• Before: uv run pytest policyengine_uk/tests/test_threshold_indexation.py -rxX on main gives 86 passed, 1 xpassed ([2023]).
• After: the same command gives 87 passed.
• uvx ruff format --check
• Changelog fragment in changelog.d/

axiom: n/a: test-only change; the LPL parameter fix is #2058's

🤖 Generated with Claude Code

[MaxGhenis]

Hand-off to the UK hub from W2 of the Autumn Budget CI plan (session local_6c1ab389).

• Head: 7f7fc5d19daac65e7fe0c5fca4d64b7ef23ff6d3, on main 6d89338.
• Review: an independent Opus 5.5 reviewer approved at this head (VERDICT: APPROVE). It confirmed class_4/thresholds/lower_profits_limit.yaml has 12,570 from 2023-04-06 (NICA 2022 s2), and its one finding, the repeated Class 4 lower profits limit is £11,908 in 2023-24; NICA 2022 s.2 sets £12,570 from 6 April 2023 #1968 sentence in test_ni_threshold_freeze.py, is fixed in 7f7fc5d.
• CI: Lint and the four smoke-imports pass; Test is pending. If it fails only on test_dataframe_dataset.py::test_an_exported_frame_loads_back_and_exports_itself, that failure is already on main (run 38143054188), not from this PR; re-run once main's fix lands.
• Order: independent of Split the CI Test job into shards and cut its runtime #2269 and Publish only the policyengine-uk release files whose checks passed #2306. Land it before Publish only the policyengine-uk release files whose checks passed #2306, whose stricter test check rejects the XPASS this removes.
• Follow-ups owed: none.