feat: add Prepare configuration for summarization middleware

[mrh997]

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[codecov]

Codecov Report

❌ Patch coverage is 55.68182% with 39 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alpha/08@3532b64). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
adk/middlewares/summarization/summarization.go	56.09%	30 Missing and 6 partials ⚠️
adk/middlewares/summarization/prompt.go	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/08     #779   +/-   ##
===========================================
  Coverage            ?   79.97%           
===========================================
  Files               ?      146           
  Lines               ?    15672           
  Branches            ?        0           
===========================================
  Hits                ?    12533           
  Misses              ?     2202           
  Partials            ?      937           

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