test(events): prove an event on the maximum is rejected after the run-context append [IFC-3008]

[ogenstad]

Why

Follow-up to #10242, taking a review suggestion from @polmichel that arrived after it merged.

That PR covered the related-resource budget from one side only: an event built on the budget still
validates after Prefect's events worker enlarges it in place. On its own that assertion would pass
just as well against a mechanism that never rejects anything, so it does not demonstrate that the
reservation earns its place.

Goal: pair it with the control that shows the failure being prevented is real.

Non-goals: no production code changes, and no revisiting of how the reservation is sized.

What changed

One new test in backend/tests/unit/event/test_limits.py. An event built on the maximum rather
than the budget has no room for the in-place run-context append, so the enlarged event lands above
the limit and no longer validates:

assert len(event.related) > get_prefect_max_related_resources()
with pytest.raises(ValidationError, match=rf"The maximum number of related resources is {case.configured_max}"):
    Event.model_validate(event.model_dump())

It reuses SURVIVAL_CASES, so it runs at 500 / 200 / 100 - the proportional branch of the
reservation, the point where the two are equal, and the floor branch - matching its companion.

The match is pinned to the case's configured maximum rather than \d+, per
.agents/rules/testing-python.md: match a substring only where the variable part genuinely cannot
be pinned down, and here it can.

How to review

The two tests are deliberately near-identical - they differ only in the size the event is built at
and in whether validation is expected to succeed. That symmetry is the point, so I left the ~18
duplicated lines rather than extracting a builder. Say the word if you would rather see the shared
construction pulled into a helper.

How to test

uv run pytest backend/tests/unit/event/test_limits.py

15 tests, all passing. invoke format and invoke lint (ruff, ty, mypy) clean.

Impact & rollout

• Backward compatibility: n/a, test-only.
• Performance: n/a.
• Config/env changes: none.
• Deployment notes: none.

Checklist

• Tests added/updated
• Changelog entry added - n/a, test-only change, labelled ci/skip-changelog
• External docs updated - n/a
• Internal .md docs updated - n/a, the mechanism is already documented in
  dev/knowledge/backend/events.md
• I have reviewed AI generated content

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks

---

_{Comparing pog-event-budget-rejection-control-IFC-3008 (e330c24) with release-1.11 (a95933f)}

[cubic-dev-ai]

1 issue found across 1 file

Confidence score: 5/5

• In backend/tests/unit/event/test_limits.py, duplicating most of test_event_on_the_budget_survives_the_prefect_run_context_append creates maintenance risk: future changes may update one test path but not the other, causing inconsistent coverage and harder debugging—factor the shared setup/assertions into a helper or parameterized test to keep behavior aligned.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="backend/tests/unit/event/test_limits.py">

<violation number="1" location="backend/tests/unit/event/test_limits.py:107">
P3: This new test duplicates nearly the entire body of `test_event_on_the_budget_survives_the_prefect_run_context_append` (both event-construction list, the `event.related +=` append, and the trailing len/validation logic). The only real differences are `get_prefect_max_related_resources()` vs `get_related_resource_budget()` as the build size and whether validation is expected to raise. Keeping the ~35 duplicated lines in sync means a change to the append or event shape must be applied twice, and the two tests drift apart silently. Extract a shared builder/helper that takes the build size, then have each test call it and assert its own outcome.</violation>
</file>

<sub>Shadow auto-approve: would not auto-approve because issues were found.

Re-trigger cubic</sub>

[cubic-dev-ai]

P3: This new test duplicates nearly the entire body of test_event_on_the_budget_survives_the_prefect_run_context_append (both event-construction list, the event.related += append, and the trailing len/validation logic). The only real differences are get_prefect_max_related_resources() vs get_related_resource_budget() as the build size and whether validation is expected to raise. Keeping the ~35 duplicated lines in sync means a change to the append or event shape must be applied twice, and the two tests drift apart silently. Extract a shared builder/helper that takes the build size, then have each test call it and assert its own outcome.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At backend/tests/unit/event/test_limits.py, line 107:

<comment>This new test duplicates nearly the entire body of `test_event_on_the_budget_survives_the_prefect_run_context_append` (both event-construction list, the `event.related +=` append, and the trailing len/validation logic). The only real differences are `get_prefect_max_related_resources()` vs `get_related_resource_budget()` as the build size and whether validation is expected to raise. Keeping the ~35 duplicated lines in sync means a change to the append or event shape must be applied twice, and the two tests drift apart silently. Extract a shared builder/helper that takes the build size, then have each test call it and assert its own outcome.</comment>

<file context>
@@ -100,3 +101,36 @@ def test_event_on_the_budget_survives_the_prefect_run_context_append(
+
+
+@pytest.mark.parametrize("case", [pytest.param(case, id=case.name) for case in SURVIVAL_CASES])
+def test_event_on_the_maximum_is_rejected_after_the_prefect_run_context_append(
+    case: SurvivalCase, monkeypatch: pytest.MonkeyPatch
+) -> None:
</file context>