chore: port an api as experiment

[asukaminato0721]

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Summary

part of #35141

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• 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

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[Copilot]

Pull request overview

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Ports the completion task “stop” endpoint to a new canonical API shape (DELETE on task resource) while keeping the legacy POST /stop route for backward compatibility.

Changes:

• Introduced CompletionTaskApi using DELETE /completion-messages/<task_id> to stop a running task.
• Refactored stop logic into a shared helper and marked the legacy POST stop route as deprecated.
• Updated unit tests to cover the new DELETE route and verify AppTaskService.stop_task arguments.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 4 comments.

File	Description
api/controllers/web/completion.py	Adds shared stop helper, introduces DELETE task route, and deprecates legacy POST /stop endpoint.
api/tests/unit_tests/controllers/web/test_completion.py	Adds/updates unit tests for the new DELETE endpoint and legacy stop behavior.

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[Copilot]

The legacy deprecated POST endpoint is tested for success, but there’s no longer a test asserting it raises NotCompletionAppError on a non-completion app mode (similar to what TestCompletionTaskApi.test_wrong_mode_raises does for DELETE). Adding that negative-path test would help prevent regression while the deprecated endpoint remains supported.

[github-actions]

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