fix: detect_file_encodings TypeError: tuple indices must be integers or slices, not str

[zhaobingshuang]

…or slices, not str #29588

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Fixes #29588

Summary

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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 @zhaobingshuang, 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 TypeError encountered in the file encoding detection logic within the RAG extractor. The issue stemmed from an attempt to access properties of an object using string keys when it was implicitly treated as a tuple or list. The fix involves a direct change to how encoding information is structured, ensuring it is consistently handled as a dictionary.

Highlights

• Bug Fix: Resolved a TypeError that occurred during file encoding detection, specifically 'tuple indices must be integers or slices, not str'.
• Code Refinement: The FileEncoding object instantiation in read_and_detect function has been replaced with a dictionary literal to correctly represent encoding details and prevent the TypeError.

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