feat(adk): improve summarization middleware user messages truncation

[mrh997]

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• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

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[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alpha/08@5b434eb). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
adk/middlewares/summarization/summarization.go	93.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/08     #769   +/-   ##
===========================================
  Coverage            ?   79.76%           
===========================================
  Files               ?      146           
  Lines               ?    15768           
  Branches            ?        0           
===========================================
  Hits                ?    12578           
  Misses              ?     2213           
  Partials            ?      977           

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