fix(client): make llm chat service works with pydantic v2

[jialeicui]

Description

• FastAPI uses return type annotation to generate router, the typing system does not support |
• Pydantic v2 optional field should specify = None if the construct function does not always contain the field.
• Top K to int

Modules

• UI
• Controller
• Agent
• Client
• Python-SDK
• Others

Checklist

• run code format and lint check
• add unit test
• add necessary doc

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (17b8f60) 82.72% compared to head (ba4adb0) 91.91%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3027      +/-   ##
============================================
+ Coverage     82.72%   91.91%   +9.18%     
============================================
  Files           558      126     -432     
  Lines         30603    15044   -15559     
  Branches       1835        0    -1835     
============================================
- Hits          25315    13827   -11488     
+ Misses         4479     1217    -3262     
+ Partials        809        0     -809     

Flag	Coverage Δ
console	?
controller	?
standalone	91.91% <100.00%> (-0.04%)	⬇️
unittests	91.91% <100.00%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tianweidut]

LGTM