fix: finish reason should be tool calls when the model responded with a tool call

[hustxiayang]

Description

Finish reason should be tool calls if the model returns a tool call response. In vertex api, there is no tool call finish reason, thus need a work around to make it compatible.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.97%. Comparing base (40fa3d6) to head (0fac71b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1486      +/-   ##
==========================================
+ Coverage   83.95%   83.97%   +0.01%     
==========================================
  Files         143      143              
  Lines       12618    12622       +4     
==========================================
+ Hits        10594    10599       +5     
+ Misses       1408     1406       -2     
- Partials      616      617       +1     

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