feat(adk): add summarization middleware

[mrh997]

Description

Add a middleware that automatically summarizes conversation history when token count exceeds the configured threshold.

Example

maxTokens := 8000
middleware := summarization.New(ctx, &summarization.Config{
    Model:        chatModel,
    TokenCounter: tokenCounter,
    Trigger: summarization.TriggerCondition{
        MaxTokens: &maxTokens,
    },
})

[codecov]

Codecov Report

❌ Patch coverage is 74.55621% with 86 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alpha/08@b1fbde2). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
adk/middlewares/summarization/summarization.go	76.59%	47 Missing and 19 partials ⚠️
adk/middlewares/summarization/prompt.go	64.28%	14 Missing and 6 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/08     #729   +/-   ##
===========================================
  Coverage            ?   80.43%           
===========================================
  Files               ?      137           
  Lines               ?    14369           
  Branches            ?        0           
===========================================
  Hits                ?    11557           
  Misses              ?     1930           
  Partials            ?      882           

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