fix delete conversations via Api and delete conversations from db as well

[jubinsoni]

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Summary

Fix #22868
close #22445

Screenshots

Before	After
...	...

before
eventough we have deleted conversation, it still used to show in log pages and entry is also there in db

after
if we delete conversation, it will not show in log page and entry in db will also be deleted

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 dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[Copilot]

Pull Request Overview

This PR fixes the conversation deletion functionality by implementing proper database cleanup when conversations are deleted via the API. Previously, conversations were only marked as deleted without removing related data or entries from the database.

• Replaces soft deletion with hard deletion for conversations and all related entities
• Adds a new method get_conversation_for_deletion to handle deletion-specific conversation retrieval
• Updates console controllers to use the centralized conversation service for deletion operations

Reviewed Changes

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

File	Description
api/services/conversation_service.py	Implements hard deletion of conversations with cascade deletion of related entities and adds deletion-specific conversation retrieval
api/controllers/console/app/conversation.py	Updates both completion and chat conversation controllers to use centralized deletion service and filters out deleted conversations from listings

[crazywoola]

@jubinsoni Sorry, try to fix this, but it failed :) Please resolve it again thanks :)

[jubinsoni]

@crazywoola
I tried couple of time to fix the for loop to delete records, some how pytest kept giving error that the table migh not have conversation_id

so to fix that I have unwrapped the for loop

also I have closed the session in the finally block so that connection goes back to connection pool

[jubinsoni]

sample celery logs

[LinzhouWang1]

我在这里有个疑问？为什么要将软删除改为硬删除？软删除不应该是一个应用通用的标准么？