fix(adk): return modified message from AfterChatModel

[hi-pender]

Summary

• Fix AfterChatModel hook to return the potentially modified message from
  state.Messages instead of the original input
• This aligns the behavior with BeforeChatModel, which correctly returns
  state.Messages after middleware execution
• Previously, modifications made to state.Messages in AfterChatModel hooks were
  ignored

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.60%. Comparing base (9e6963f) to head (0552102).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #717   +/-   ##
=======================================
  Coverage   81.59%   81.60%           
=======================================
  Files         127      127           
  Lines       12253    12253           
=======================================
+ Hits         9998     9999    +1     
  Misses       1512     1512           
+ Partials      743      742    -1     

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