feat: settle delete behavior for the competency criteria models

[jesperhodge]

Part 8 of 8 for issue openedx#641. Based on part 7. Parts 4, 6, and 7 each declare their own on_delete values now, as they add the foreign key that value governs. This PR adds no on_delete value of its own; it adds the tests that exercise those decisions together, one cross-model limitation those decisions produce, and the tests pinning the MySQL collector behavior CompetencyRuleProfile.scope_code's design depends on.

#	Branch	What it adds
1	cbe-641-01-adr	ADR-0002 scope_code amendment (openedx#809)
2	cbe-641-02-deps-and-layering	django-simple-history, .importlinter (#2)
3	cbe-641-03-taxonomy-overrides-org	taxonomy_overrides_org, models/ package (#3)
4	cbe-641-04-criteria-group	CompetencyCriteriaGroup (#4)
5	cbe-641-05-rule-payloads	RuleType, validate_rule_payload (#5)
6	cbe-641-06-rule-profile	CompetencyRuleProfile + seed (#6)
7	cbe-641-07-criterion	CompetencyCriterion (#7)
8	cbe-641-08-delete-behavior	this PR — the cross-model delete tests, one known limitation, and the MySQL-collector tests

Where each decision lives

Each of the nine foreign keys below is declared, with its real on_delete value, in the part that adds it. This PR adds none of them; it is a reference for reading the tests that follow.

Foreign key	Value	Declared in
CompetencyCriteriaGroup.parent	CASCADE	part 4 (#4)
CompetencyCriteriaGroup.tag	CASCADE	part 4 (#4)
CompetencyCriteriaGroup.course	CASCADE	part 4 (#4)
CompetencyRuleProfile.organization	PROTECT	part 6 (#6)
CompetencyRuleProfile.course	CASCADE	part 6 (#6)
CompetencyRuleProfile.competency_taxonomy	CASCADE	part 6 (#6)
CompetencyCriterion.group	CASCADE	part 7 (#7)
CompetencyCriterion.object_tag	CASCADE	part 7 (#7)
CompetencyCriterion.rule_profile	RESTRICT	part 7 (#7)

Each part above explains its own values and, where relevant, the AC deviations and open questions attached to them. on_delete expresses containment, not protection: it governs deletion of the row a foreign key points at, never the row holding it. The seven cascading edges are how Django's collector walks down the tree, and they are load-bearing rather than a relaxation: openedx#642 puts PROTECT on the learner status tables one and two levels below the tag, and the collector reaches those only along edges marked CASCADE.

One deviation from openedx#641's acceptance criteria that belongs to this PR

ProtectedError tests that AC 27 does not ask for. AC 27 says only the cascade half is tested here, because the ProtectedError half needs openedx#642's tables. This PR tests three ProtectedError cases anyway: an organization with a scoped profile, a profile referenced by a criterion, and the known limitation below. None needs a status row, so they cost nothing and pin real behavior. Additive, not a violation.

One known limitation, pinned rather than worked around

Deleting a CompetencyTaxonomy whose taxonomy-scoped profile is assigned to a CompetencyCriterion raises ProtectedError naming the criterion, not the profile actually being cascaded away, because PROTECT checks the database rather than the collector's own pending set. That criterion would also be cascade-deleted in the same operation through the tag chain, so the failure is spurious.

It is unreachable in this phase: no code path creates a taxonomy-scoped profile. ADR-0002 Decision 7 records the fix for when scoped profiles are built, a fifth reassignment event on Decision 4. test_taxonomy_delete_blocked_by_its_scoped_profile_names_the_criterion_not_the_profile pins the current behavior so that changing it later is deliberate.

Reviewing this PR

test_criteria_deletion.py opens with the table above, then works through each model's foreign keys, the transitive cases openedx#641 requires, the known limitation, and finally the MySQL collector path. test_criteria_trees.py holds the two integrative tests, which are where a per-foreign-key test cannot help you:

• test_deleting_a_middle_group_removes_its_subtree_but_leaves_the_rest_of_the_tree_untouched builds a tree with a surviving sibling branch and a mix of profile-assigned and override-governed criteria, then asserts the exact surviving row set. A test that only checks "the deleted branch is gone" cannot distinguish a correct cascade from one that over-deletes into a sibling.
• test_object_tag_delete_leaves_a_childless_criteria_group_behind pins an accepted outcome, not a bug: CompetencyCriteriaGroup never references ObjectTag, so nothing gives the collector a reason to reach the group, and it is left inert and childless.

The three ..._under_mysql_collector_semantics tests monkeypatch can_defer_constraint_checks to False. Without that, the code path they exercise never runs on SQLite and they pass against broken and correct code alike. That path is also why CompetencyRuleProfile.scope_code is a plain column rather than a GeneratedField, reasoned through in part 6 (#6).

What is deliberately not here

Every "raises ProtectedError because a learner status row exists" case. Those need openedx#642's three Student*Status tables, and nothing here stubs or fakes a status model to stand in. Until openedx#642 merges, main carries a cascade chain with no PROTECT at the bottom, so deleting a tag removes the whole authored tree and nothing objects. That window is expected and harmless, because the learner status tables do not exist yet.

Also not here: any delete() override, archive-versus-delete branch, or deletion-lock field. openedx#655 governs that and it lands in openedx#674, openedx#675, openedx#716, openedx#776 and openedx#778.

Verifying

pytest tests/openedx_learning --no-cov -q                              # 98 passed
pytest tests/openedx_learning --no-cov -q --ds=mysql_test_settings
python manage.py makemigrations openedx_learning --check --dry-run     # no changes detected

By the time part 7 lands, every one of the nine values above is already real; there is nothing left "undecided" going into this PR. What this PR adds is the cross-model and edge-case coverage a per-field test cannot give you, such as the two integrative tests above and the known limitation below.

Refs openedx#641

[jesperhodge]

Closing this one. Every test here turned out to only need the model(s) that already exist by the time it could be written: the MySQL-collector tests split between whichever model's course/competency_taxonomy foreign key they exercise (parts 4 and 6), and everything needing CompetencyCriterion — including the transitive tag/taxonomy cascades, the two RESTRICT payoff/residual scenarios, and the one tree-wide integration test — moved to part 7, which is what completes the tree.

So there was nothing left that actually needed a PR of its own on top of part 7. All 21 tests now live in:

• test_criteria_group_deletion.py (part 4, feat: add CompetencyCriteriaGroup, the criteria tree's AND/OR node #4)
• test_rule_profile_deletion.py (part 6, feat: add CompetencyRuleProfile and seed the system default #6)
• test_criterion_deletion.py and test_criteria_trees.py (part 7, feat: add CompetencyCriterion, the criteria tree's leaf #7)

Part 7 is now the top of the stack. Nothing in this branch is lost — it's superseded, not discarded.