fix: dispatch dataset cleanup when doc_form is missing

[Zhengcy05]

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Summary

Fixes #38537

This PR fixes the dataset deletion path for migrated datasets whose doc_form is missing.

The delete event handler previously returned early when either doc_form or indexing_technique was empty, which meant clean_dataset_task was never dispatched for some migrated datasets. However, clean_dataset_task already handles an empty doc_form by falling back to PARAGRAPH_INDEX, so the handler was blocking a case the downstream cleanup path could already recover from.

This change keeps the existing safeguard for missing indexing_technique, but stops skipping cleanup when only doc_form is missing.

Changes included in this PR:

• narrow the dataset deletion guard in clean_when_dataset_deleted.py
• update the existing integration test expectation for the doc_form=None case
• add a focused unit test for the event handler dispatch behavior

Local verification:

• uv run --project api --group dev pytest api/tests/unit_tests/events/event_handlers/test_clean_when_dataset_deleted.py
• uv run --project api --group dev ruff check api/events/event_handlers/clean_when_dataset_deleted.py api/tests/unit_tests/events/event_handlers/test_clean_when_dataset_deleted.py api/tests/test_containers_integration_tests/services/test_dataset_service_delete_dataset.py

Screenshots

Before	After
N/A	N/A

Checklist

• This change requires a documentation update, included: [Dify Document](https://github.com/langgenius/dify-docs)
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Diff

base → PR

--- /tmp/pyrefly_base.txt	2026-07-09 04:00:56.813213340 +0000
+++ /tmp/pyrefly_pr.txt	2026-07-09 04:00:48.392145449 +0000
@@ -1,3 +1,5 @@
+ERROR Argument `str | None` is not assignable to parameter `doc_form` with type `str` in function `celery.app.task.Task.delay` [bad-argument-type]
+  --> events/event_handlers/clean_when_dataset_deleted.py:20:9
 ERROR Argument `SimpleNamespace` is not assignable to parameter `dataset` with type `Dataset` in function `dify_vdb_alibabacloud_mysql.alibabacloud_mysql_vector.AlibabaCloudMySQLVectorFactory.init_vector` [bad-argument-type]
   --> providers/vdb/vdb-alibabacloud-mysql/tests/unit_tests/test_alibabacloud_mysql_factory.py:42:38
 ERROR Argument `SimpleNamespace` is not assignable to parameter `dataset` with type `Dataset` in function `dify_vdb_alibabacloud_mysql.alibabacloud_mysql_vector.AlibabaCloudMySQLVectorFactory.init_vector` [bad-argument-type]

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	52.59%	52.59%	0.00%
Strict coverage	52.11%	52.11%	0.00%
Typed symbols	32,538	32,538	0
Untyped symbols	29,605	29,605	0
Modules	2982	2982	0

[fatelei]

style check failed

[Zhengcy05]

style check failed

Thx, I've fixed it.