fix(adk): support after chatmodel if no tools configured

[meguminnnnnnnnn]

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[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.25%. Comparing base (d1d1035) to head (f2ccc2f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
adk/chatmodel.go	77.77%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #643      +/-   ##
==========================================
+ Coverage   80.19%   80.25%   +0.06%     
==========================================
  Files         124      124              
  Lines       11818    11829      +11     
==========================================
+ Hits         9477     9493      +16     
+ Misses       1614     1610       -4     
+ Partials      727      726       -1     

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