fix(adk): preserve nil agentCancelOpts in stopSignal.check to prevent UntilIdleFor from canceling agent

[shentongmartin]

Problem

Stop(UntilIdleFor(...)) spuriously cancels the running agent instead of deferring the stop until idle.

Expected: Stop(UntilIdleFor(30s)) → agent finishes → idle timer ticks → loop exits
Actual:   Stop(UntilIdleFor(30s)) → agent canceled immediately → loop exits

Solution

Two fixes in stopSignal.check() and Stop():

1. Nil-preservation in check(): append([]AgentCancelOption{}, nil...) returns []AgentCancelOption{} (non-nil empty slice), not nil. The watchStopSignal goroutine's tryCancel uses opts == nil to distinguish "no cancel intent" from "CancelImmediate" (empty opts). Added a nil guard so check() returns nil when agentCancelOpts is nil.

2. Drop cancel opts when paired with UntilIdleFor: Stop(UntilIdleFor(d), WithImmediate()) is semantically contradictory — "wait for idle" + "cancel now". Stop() now clears agentCancelOpts when idleFor > 0. To escalate, users must issue a separate Stop(WithImmediate()) call.

Key Insight

UntilIdleFor operates on a different axis than cancel options. It controls when the loop exits (after idle duration), while cancel options control how the running agent is interrupted. Combining them in the same Stop() call is meaningless: if you're waiting for idle, there's no running agent to cancel. Cancel opts only make sense in a follow-up Stop() call that overrides the idle wait:

loop.Stop(UntilIdleFor(30 * time.Second))  // wait for idle
// ... later, if you need to abort:
loop.Stop(WithImmediate())                 // overrides idle, cancels agent

---

问题

Stop(UntilIdleFor(...)) 会错误地取消正在运行的 agent，而非等待空闲后再停止。

期望：Stop(UntilIdleFor(30s)) → agent 完成 → 空闲计时器计时 → 循环退出
实际：Stop(UntilIdleFor(30s)) → agent 被立即取消 → 循环退出

解决方案

在 stopSignal.check() 和 Stop() 中进行了两处修复：

1. check() 中的 nil 保留：append([]AgentCancelOption{}, nil...) 返回非 nil 的空切片 []AgentCancelOption{}。watchStopSignal 的 tryCancel 通过 opts == nil 区分"无取消意图"和"CancelImmediate"（空 opts）。添加了 nil 判断，使 check() 在 agentCancelOpts 为 nil 时直接返回 nil。

2. UntilIdleFor 搭配 cancel opts 时丢弃后者：Stop(UntilIdleFor(d), WithImmediate()) 语义矛盾——"等待空闲" + "立即取消"。Stop() 现在在 idleFor > 0 时清除 agentCancelOpts。要升级停止方式，用户需发起单独的 Stop(WithImmediate()) 调用。

核心洞察

UntilIdleFor 与 cancel 选项处于不同的语义轴。它控制的是循环何时退出（空闲一段时间后），而 cancel 选项控制的是如何中断正在运行的 agent。在同一个 Stop() 调用中组合它们没有意义：如果你在等待空闲，就没有正在运行的 agent 需要取消。Cancel opts 只在覆盖空闲等待的后续 Stop() 调用中才有意义。

[codecov]

Codecov Report

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Additional details and impacted files

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  Files               ?      162           
  Lines               ?    20593           
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  Hits                ?    16946           
  Misses              ?     2458           
  Partials            ?     1189           

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