feat(adk): tool reduction middleware

[N3kox]

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重构 ADK 工具上下文压缩中间件（Tool Reduction Middleware）

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en:

This PR introduces a tool context reduction middleware in ADK that supports per-tool configurable truncation of tool outputs (by max content length and max line length) for both invokable and streamable tools, and a token-based offload mechanism that, once the estimated tokens of conversation messages exceed a configurable threshold, offloads full tool results to a pluggable backend (with a default in-memory backend and /tmp file-path strategy) while replacing in-context content with a short reference string, along with utilities such as default tokenizer, default offload handler, and comprehensive unit tests covering truncation, offload, and default middleware construction behaviors.

zh(optional):

本 PR 在 ADK 中新增 context 压缩 middleware，支持按工具配置输出截断策略（包括按整体长度和单行长度截断），同时在对话消息估算 token 数超过可配置阈值时，对工具调用结果进行自动卸载 (offloading)，将完整结果写入可插拔后端存储（默认使用内存文件系统与 "/tmp" 路径），并在上下文中以短提示文案替代原始内容；同时提供默认分词器、默认 offload 处理逻辑、默认压缩 middleware 配置，并补充了覆盖截断逻辑、offload 流程以及默认中间件构造的单元测试

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[codecov]

Codecov Report

❌ Patch coverage is 61.21884% with 140 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alpha/08@a74b18b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
adk/middlewares/reduction/reduction.go	60.11%	105 Missing and 29 partials ⚠️
adk/middlewares/reduction/consts.go	75.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/08     #746   +/-   ##
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  Coverage            ?   80.04%           
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  Files               ?      145           
  Lines               ?    15266           
  Branches            ?        0           
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  Hits                ?    12220           
  Misses              ?     2083           
  Partials            ?      963           

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[mrh997]

300k 是不是定太大了，现在的模型上下文窗口基本 200k 以下

[N3kox]

改成200k了