chore: hide model provider setting in default model setting

[iamjoel]

Also improve the copywriting of reasoning model tooltip.

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 make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.98%. Comparing base (b04b444) to head (7a1f203).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #36383      +/-   ##
==========================================
- Coverage   85.99%   85.98%   -0.01%     
==========================================
  Files        4458     4458              
  Lines      211918   211898      -20     
  Branches    39544    39535       -9     
==========================================
- Hits       182232   182211      -21     
- Misses      26471    26472       +1     
  Partials     3215     3215              

Flag	Coverage Δ
dify-ui	94.80% <ø> (ø)
web	86.56% <100.00%> (-0.01%)	⬇️

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