fix(api): clean multimodal attachments in batch deletion

[hieheihei]

Summary

Fixes #41255

Clean binding-backed multimodal attachments during batch document deletion. The task now collects attachment IDs from SegmentAttachmentBinding, reuses the existing UploadFile and storage cleanup, and deletes bindings with their segments.

Validated with focused task tests (3 passed), make lint, make type-check, and a two-document browser E2E test.

Screenshots

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 vp staged (frontend) to appease the lint gods — frontend not changed

From Codex

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	61.14%	61.14%	+0.00%
Strict coverage	60.73%	60.73%	+0.00%
Typed symbols	41,940	41,942	+2
Untyped symbols	26,838	26,839	+1
Modules	3248	3248	0