fix(adk): fix concurrent compile race in ChatModelAgent

[shentongmartin]

Summary

Problem	Solution
Data race when multiple requests call chain.Compile() concurrently on shared chain/graph instances in ChatModelAgent	Move chain and inner graph creation inside closures so each request has its own instance

Problem

In ChatModelAgent, both buildNoToolsRunFunc and buildReactRunFunc create a shared chain (and inner graph g for react) outside the closure, then call chain.Compile() inside. When multiple requests execute concurrently, they race on:

1. chain.addEndIfNeeded() - modifies hasEnd flag
2. graph.compile() - modifies internal state like compiled, handlerPreNode
3. For react: innerGraph.compile() - nested graph also gets compiled with races

Solution

Create chain and inner graph per request inside the closure:

// Before (race condition)
g, err := newReact(ctx, conf)  // shared
chain := NewChain[...]().AppendGraph(g)  // shared
return func(...) {
    runnable, _ := chain.Compile(...)  // RACE
}

// After (safe)
return func(...) {
    g, err := newReact(ctx, conf)  // per request
    chain := NewChain[...]().AppendGraph(g)  // per request
    runnable, _ := chain.Compile(...)  // safe
}

Key Insight

Neither Chain nor Graph in the compose package supports concurrent Compile() calls on the same instance. The compile operation modifies internal state without synchronization. Creating instances per request is the safe pattern when each request may have different compile options (like WithCheckPointStore(store)).

---

概述

问题	解决方案
多个请求并发调用共享 chain/graph 实例的 chain.Compile() 时存在数据竞争	将 chain 和内部 graph 的创建移入闭包内，使每个请求拥有独立实例

问题

在 ChatModelAgent 中，buildNoToolsRunFunc 和 buildReactRunFunc 都在闭包外创建共享的 chain（react 模式还有内部 graph g），然后在闭包内调用 chain.Compile()。当多个请求并发执行时，会在以下位置产生竞争：

1. chain.addEndIfNeeded() - 修改 hasEnd 标志
2. graph.compile() - 修改内部状态如 compiled、handlerPreNode
3. React 模式：innerGraph.compile() - 嵌套的 graph 也会被编译并产生竞争

解决方案

在闭包内为每个请求创建 chain 和内部 graph。

关键洞察

compose 包中的 Chain 和 Graph 都不支持在同一实例上并发调用 Compile()。编译操作会修改内部状态而没有同步机制。当每个请求可能有不同的编译选项时（如 WithCheckPointStore(store)），为每个请求创建新实例是安全的模式。

[codecov]

Codecov Report

❌ Patch coverage is 68.18182% with 7 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alpha/08@f887fed). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
adk/chatmodel.go	68.18%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/08     #775   +/-   ##
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  Coverage            ?   79.79%           
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  Files               ?      146           
  Lines               ?    15844           
  Branches            ?        0           
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  Hits                ?    12642           
  Misses              ?     2218           
  Partials            ?      984           

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