frontend: Fix Agent model being set

[tianjing-li]

AI Description

This pull request updates the useState hook in the ConfigStep component to use the fields.model value instead of the DEFAULT_AGENT_MODEL constant.

• The useState hook now initializes the selectedValue state with the value of fields.model.
• This change allows the component to dynamically set the initial value of the selectedValue state based on the fields prop, providing more flexibility in setting the default agent model.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.87%. Comparing base (7e2e993) to head (a70dfe4).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #819      +/-   ##
==========================================
+ Coverage   79.80%   79.87%   +0.07%     
==========================================
  Files         243      244       +1     
  Lines       10355    10436      +81     
==========================================
+ Hits         8264     8336      +72     
- Misses       2091     2100       +9     

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[malexw]

Good fix!