fix(adk): preserve checkpoints for canceled tool streams - #1303

Problem

On v0.9.19, CancelImmediate with WithRecursive can terminate an AgentTool child while its ChatModelAgent is consuming a StreamableTool result. The resulting ErrStreamCanceled escapes as an ordinary stream failure during checkpoint materialization, so the child finishes without resumable ToolNode interrupt state. The caller then observes ErrExecutionEnded, and Runner.Resume cannot continue from the interrupted tool call.

Solution

Treat framework-driven tool stream cancellation as checkpoint control flow:

• identify cancellation consistently across the ADK and compose boundary;
• retain completed parallel tool results while marking canceled calls for rerun;
• convert the canceled tool stream into a composite interrupt carrying ToolNode input and rerun state;
• merge that nested interrupt into the graph checkpoint instead of serializing the canceled stream as output.

This is a v0.9 backport of #1224 (0fb0380e) with an end-to-end regression covering AgentTool -> ChatModelAgent -> StreamableTool cancellation and resume.

Key Insight

A framework-canceled tool stream is not a terminal tool failure. Its partial output cannot be treated as completed state; the checkpoint must preserve the original ToolNode input, retain successfully completed sibling calls, and rerun only canceled calls on resume.

问题

在 v0.9.19 中，当 AgentTool 内部的 ChatModelAgent 正在消费 StreamableTool 结果时，使用 CancelImmediate + WithRecursive 可能让 ErrStreamCanceled 在 checkpoint 物化阶段作为普通流错误返回。child 因此结束，但没有生成可恢复的 ToolNode interrupt state；调用方收到 ErrExecutionEnded，Runner.Resume 也无法从被中断的工具调用继续。

解决方案

将框架触发的工具流取消作为 checkpoint 控制流处理：

• 在 ADK 与 compose 边界统一识别 stream cancellation；
• 保留已经完成的并行工具结果，并将被取消的调用标记为 rerun；
• 把 canceled tool stream 转换为携带 ToolNode 输入和 rerun state 的 composite interrupt；
• 将该 nested interrupt 合并到 graph checkpoint，而不是把已取消的 stream 当作输出序列化。

这是 #1224（0fb0380e）面向 v0.9 的 backport，并补充了 AgentTool -> ChatModelAgent -> StreamableTool 取消与恢复的端到端回归覆盖。

关键结论

框架取消的工具流不是终态工具失败。其部分输出不能被视为已完成状态；checkpoint 必须保留原始 ToolNode 输入和已完成的 sibling tool 结果，并在恢复时只重跑被取消的调用。