fix(kb): clean up documents and media when deleting a knowledge base

[he-yufeng]

KnowledgeBaseManager.delete_kb() deletes the KnowledgeBase row and the vector DB, but the KB's KBDocument and KBMedia rows carry kb_id with no ORM relationship or foreign-key cascade. Deleting a knowledge base therefore leaves all of its documents and media orphaned in the SQLite database.

This is the same cascade gap #9120 fixed for single-document deletion (delete_document_by_id deletes KBMedia explicitly because a bulk DELETE does not trigger ORM cascade) — the whole-KB delete path was the missed sibling.

删除知识库时只删了 KnowledgeBase 行和向量库，但该库的 KBDocument / KBMedia 通过 kb_id 关联、没有 ORM/外键级联，会把全部文档和多媒体记录留成孤儿（与 #9120 修的单文档删除是同一类问题）。

Modifications / 改动点

• Add KBSQLiteDatabase.delete_kb_by_id(kb_id) that bulk-deletes KBMedia, KBDocument, and the KnowledgeBase row for the KB, mirroring delete_document_by_id.

• delete_kb() calls it (after delete_vec_db()) instead of only session.delete(kb_helper.kb).

• Unit tests in tests/unit/test_kb_document_cleanup.py: whole-KB deletion leaves no orphaned document/media, and deleting one KB does not touch another KB's rows.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

$ .venv/bin/python -m pytest tests/unit/test_kb_document_cleanup.py -q
6 passed

Red-before-green check: with the two child-table deletes removed from delete_kb_by_id, test_delete_kb_cleans_documents_and_media fails because the KBDocument/KBMedia rows survive the KB deletion; with the fix in place they are gone.

Checklist / 检查清单

• 👀 Well-tested (unit tests + red-before-green verification).
• 🤓 No new dependencies.
• 😮 No malicious code.

Summary by Sourcery

Ensure deleting a knowledge base also removes its associated documents and media, avoiding orphaned rows in SQLite.

Bug Fixes:

• Fix knowledge base deletion leaving orphaned KBDocument and KBMedia records in the SQLite database.
• Ensure deletion of one knowledge base does not affect documents and media belonging to other knowledge bases.

Enhancements:

• Add a KBSQLiteDatabase.delete_kb_by_id helper to encapsulate deletion of a knowledge base and its related documents and media.
• Update KnowledgeBaseManager.delete_kb to use the new database helper for consistent cleanup behavior.

Tests:

• Extend kb document cleanup unit tests to cover whole-knowledge-base deletion and isolation of data across multiple knowledge bases.

[sourcery-ai]

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[gemini-code-assist]

Code Review

This pull request introduces a mechanism to clean up associated documents and multimedia records when a knowledge base is deleted, preventing orphan records in the database. Specifically, it adds a delete_kb_by_id method to kb_db_sqlite.py to delete KBMedia, KBDocument, and KnowledgeBase records in a single transaction, updates kb_mgr.py to use this new method, and adds corresponding unit tests to verify the cleanup behavior. There are no review comments, so I have no additional feedback to provide.

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