Tag import: parent errors name their row and report every cycle as a cycle

This section is background and a suggested approach, not the source of truth. The User Story and Acceptance Criteria are what must be true when the work is done.

In short

Naming the row in the unknown-parent error. The plan gives each action it builds from a row that row's 1-based position among the file's data rows. The stale-parent error from #858 already uses that position for its "Row N: " prefix, but the "Unknown parent tag" error, built in the same parent validation, ignores it. The change applies the "Row N: " prefix once, at the end of that validation, to whichever of the two parent errors it returns, so the two errors cannot drift apart again. Steps that no row describes (deletes, the "move to the root first" steps, and the delete phase's promotions to the root) have no row and never validate a parent, so they are unaffected.

Reporting every cycle as a cycle. Before it builds any row's action, the plan computes the tag tree as it will be after the import and walks that tree to find parent cycles. Today the walk deliberately skips a cycle made only of new tags and leaves it to the per-row "Unknown parent tag" check. The change removes that exception, so every cycle that has a row in the file gets the existing parent cycle error once, on the cycle's first row in the file.

Not reporting a cycle's rows as unknown parents. The plan builds each row after the row that creates or renames-in its parent, so that the per-row check finds the parent among the actions built so far. A cycle has no such order: one of its rows is always built before the row that creates its parent. When that parent is a new tag, the per-row check doesn't find it and reports "Unknown parent tag", although the file has a row for it. A renamed-in parent is found anyway, because the plan records every rename's new id up front, and an existing parent is found in the database. That is why only cycles through a new tag show the extra error. The fix has the cycle check record the rows of every cycle it reports, and has the per-row check skip the "Unknown parent tag" error for those rows. Skipping it hides no real problem, because every parent on a cycle is a tag of the final tree. Only that one error is skipped: the stale-parent error from #858 is still reported for a row on a cycle.

Implementation specifics

• Row prefix in ImportAction._validate_parent (src/openedx_tagging/import_export/actions.py): build either message, then prefix Row {row}: when self.row is not None, at a single return. The unknown-parent text after the prefix stays Unknown parent tag ({parent_id}). Add a row for the parent to your file.
• Every cycle in TagImportPlan._validate_hierarchy (src/openedx_tagging/import_export/import_plan.py): remove the any(pk_of[key] != NEW_TAG for key in cycle) condition but keep the rows guard, so a cycle with no row in the file is still not reported.
• Cycle rows recorded: add each reported cycle's rows, as 1-based numbers like ImportAction.row, to a set stored in self.indexed_actions["_rows_on_a_cycle"], the same channel _build_staging_actions uses for _vacated_pks and _renamed_to_ids. _validate_hierarchy runs before the row actions are built, so the set is complete when _validate_parent reads it.
• Skip in _validate_parent: return None instead of the unknown-parent error when self.row in indexed_actions.get("_rows_on_a_cycle", set()). The .get default keeps the unit tests that pass a hand-built indexed_actions working.
• Drop the pk_of parameter of _validate_hierarchy and update its call in generate_actions: nothing in the method reads it after the change, and pylint would flag it as unused. NEW_TAG stays, because _end_state and _build_detach_actions still use it.
• Docstrings: _validate_hierarchy currently says that a cycle made only of new tags is left to _validate_parent. Make it say that every cycle is reported once, on its first row, and that its rows are recorded so _validate_parent doesn't also report them as unknown parents. Add one sentence about the skip to _validate_parent.
• Contract: import_tags keeps its signature and return value. A file that was rejected before is still rejected; only the error text differs. No migration.
• Two existing tests assert the old behavior and change, both in tests/openedx_tagging/import_export/test_import_plan.py:
  • In the first data set of test_plan, the expected line becomes Action error in 'update_parent' (#5): Row 4: Unknown parent tag (tag_100). Add a row for the parent to your file.
  • Delete test_generate_actions_cycle_of_new_tags_reports_an_unknown_parent_once and move its rows into test_generate_actions_parent_cycle_rejected_once, expecting Row 1's parent_id (new_b). Add data sets to that test for a new tag that names itself, a three-tag cycle of new tags, and the renamed-plus-new cycle in both row orders.
• New unit tests in test_import_plan.py:
  • A file whose create row, move row and rename row all name tag_100 gets three errors, prefixed Row 1:, Row 2: and Row 3:.
  • A two-tag cycle of new tags plus a third row naming tag_100 gets exactly two errors, the cycle error and the Row 3: unknown-parent error.
  • The tag_1/tag_3 swap with a cycle through tag_2 gets the cycle error and the stale-parent error.
• New unit test in tests/openedx_tagging/import_export/test_actions.py: a _validate_parent case with row set that asserts the prefix. The existing case there builds its action without a row, so its expected text stays the same.
• Acceptance tests: add one test per scenario to TestImportAcceptanceScenarios in tests/openedx_tagging/import_export/test_api.py, using _assert_rejected(..., error_count=...) across _orders.
• Out of scope: the duplicate-value errors, which also name no row, and the deleted-parent check in #893.

Modified files

• Builds on the #858 pull request; start after it merges.
• Land this ticket before #893, which follows this ticket's "Row N:" form and edits the same _validate_hierarchy docstring.
• Prior art: TagImportPlan._end_state, _validate_hierarchy and _order_rows_parent_first in src/openedx_tagging/import_export/import_plan.py; ParentCycleError in src/openedx_tagging/import_export/exceptions.py.
• ADR: docs/openedx_tagging/decisions/0010-mutable-tag-external-id.rst.
• #673 (previous_id); epic #720.