fix: correct fetch_from for customizable models

[kurokobo]

Summary

This PR is for fix/chore-fix branch, as a follow-up for #12372.

This PR fixes the issue that customizable models are treated as predefined models by API which causes the missing Config button for customizable models on Web UI.

Additional context: https://discord.com/channels/1082486657678311454/1324964911356317736, #12372 (comment)

Screenshots

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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

[Yeuoly]

LGTM