Fix external knowledge Issues: (#17685)

[briqt]

Use metadata.document_id or title for external KB retrieval to fix citation grouping

Summary

Resolves #17685

When processing external knowledge base retrieval results, the document_id was not assigned, resulting in all retrieved results having a document_id of null. However, the front-end page happens to group and display cited files based on the document_id, leading to a maximum of only one file citation being displayed during the conversation.

Solution: When processing external knowledge base retrieval results, attempt to obtain metadata.document_id from the external knowledge base's returned results. If there is no document_id, then take metadata.title as the document_id.

Screenshots

Before	After

Checklist

Important

Please review the checklist below before submitting your pull request.

• 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

[JohnJyong]

LGTM