fix(adk): disable automatic instruction rendering in DeepAgent

[hi-pender]

Summary

This PR customizes the GenModelInput function for DeepAgent to disable automatic instruction rendering from session values.

Background

The default GenModelInput in ChatModelAgent automatically retrieves session values via GetSessionValues(ctx) and renders placeholders in the instruction using prompt.FromMessages. While this feature is explicitly documented and exposed in ChatModelAgent, it can be confusing for DeepAgent users who may not be aware of this implicit behavior.

Changes

• Added a custom genModelInput function in the deep package that simply combines the instruction and messages without template rendering
• Configured DeepAgent to use this custom function instead of the default one

Impact

• DeepAgent will no longer automatically render instruction placeholders from session values
• Users who need instruction rendering can still use ChatModelAgent directly with its default behavior
• All existing tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.75%. Comparing base (d970d34) to head (0db92ee).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #726      +/-   ##
==========================================
- Coverage   81.82%   81.75%   -0.08%     
==========================================
  Files         129      129              
  Lines       12416    12423       +7     
==========================================
- Hits        10160    10157       -3     
- Misses       1500     1510      +10     
  Partials      756      756              

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