[CLEAN] Synthetic Benchmark PR #30445 - refactor: more ns.model to BaseModel

[bar-qodo]

Benchmark PR langgenius#30445

Type: Clean (correct implementation)

Original PR Title: refactor: more ns.model to BaseModel
Original PR Description: > [!IMPORTANT]

1. Make sure you have read our contribution guidelines
2. Ensure there is an associated issue and you have been assigned to it
3. Use the correct syntax to link this PR: Fixes #<issue number>.

Summary

part of langgenius#28015 , move api/fields/message_fields.py and api/fields/conversation_fields.py , update all the callsites.

Screenshots

Before	After
...	...

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 dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Original PR URL: langgenius#30445