fix(agent-v2): preserve oauth2 credential refs when converting tool c…

[linw1995]

…onfig

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Summary

Screenshots

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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 85.28%. Comparing base (d9884ef) to head (4fb427d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #38303   +/-   ##
=======================================
  Coverage   85.28%   85.28%           
=======================================
  Files        4982     4982           
  Lines      263215   263220    +5     
  Branches    50015    50017    +2     
=======================================
+ Hits       224471   224478    +7     
+ Misses      34361    34359    -2     
  Partials     4383     4383           

Flag	Coverage Δ
dify-ui	94.93% <ø> (ø)
web	84.86% <100.00%> (+<0.01%)	⬆️

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