feat(adk): improve summarization prompt

[mrh997]

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[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alpha/08@a34d52f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
adk/middlewares/summarization/prompt.go	75.00%	1 Missing and 1 partial ⚠️
adk/middlewares/summarization/summarization.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/08     #774   +/-   ##
===========================================
  Coverage            ?   79.83%           
===========================================
  Files               ?      146           
  Lines               ?    15843           
  Branches            ?        0           
===========================================
  Hits                ?    12648           
  Misses              ?     2211           
  Partials            ?      984           

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