fix: fixed workflow as tool files field return empty problem

[CrabSAMA]

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Summary

Fixes https://github.com/langgenius/dify/issues/26431

Add message.meta check, which is File instance and mark it is file link message, add it into files field.

Screenshots

Before	After
files field is empty	files field have correct content

Checklist

• [ ] This change requires a documentation update, included: Dify Document
• [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[gemini-code-assist[bot]]

Summary of Changes

Hello @CrabSAMA, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the workflow tool processing where file attachments were not being correctly recognized and populated in the output's 'files' field. The changes introduce robust handling for LINK type messages, specifically looking for File instances within the message metadata. This ensures that any files passed through the workflow are properly extracted, listed, and presented to the user, thereby fixing the empty files field problem and improving the reliability of file-related operations within workflows.

Highlights

• Bug Fix: Resolved an issue where the 'files' field in workflow tool outputs was incorrectly returning empty, preventing proper display and handling of attached files.
• File Link Recognition: Implemented new logic to accurately identify and process file links within ToolInvokeMessage.MessageType.LINK messages by checking the message.meta for a File instance.
• File Inclusion: Ensured that identified File instances from message.meta are correctly appended to the files list, making them available in the workflow output.
• Output Formatting: Updated the text stream output for file links to display 'File:' instead of 'Link:', providing clearer context for users.

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