fix(adk): stop forwarding backgrounded sub-agent events to the parent turn

[hi-pender]

Summary

• A managed sub-agent run that is backgrounded (explicit run_in_background, or auto-moved to the background at the foreground deadline) runs detached and outlives the turn that launched it. It kept forwarding the inner agent's events to the parent turn's event generator, which the turn closes on end — injecting events into a stream the user has stopped watching, and racing a send against that close.
• Introduce a lifecycle signal backgroundtask.TaskInfo.Backgrounded (<-chan struct{}) that closes when a task moves to the background: before work starts for an explicit RunInBackground launch, and at the auto-background transition (detach). Foreground-completing runs never fire it.
• Add AgentTool option adk.WithAgentToolParentForwardUntil(done) that bounds parent forwarding by that signal. The subagent middleware wires the two together, so forwarding stops the moment a run detaches; foreground runs are unaffected.

Design notes

• Concerns are decoupled: backgroundtask owns the lifecycle signal (event-agnostic), adk/agent_tool owns the forwarding action + the "forward until" gate, subagent middleware is the only layer that wires them (one line in the work closure). No reading/overriding of the parent generator option, no interposed channel/pump/atomic.
• The gated (background-capable) path uses a non-panicking send by definition — it may outlive its consumer. The plain foreground AgentTool path keeps a panicking Send, so a send on an unexpectedly-closed generator still surfaces as the bug it is.
• Known, documented residual: under auto-background the detached forwarder's lifetime exceeds the parent turn; the gate narrows but cannot fully eliminate the race with the parent close. The gated path's non-panicking send is what makes that safe, and it does not affect the synchronous path.

Test plan

• go test -race ./adk/... (passes locally)
• golangci-lint run ./adk/... (0 issues locally)
• backgroundtask: signal timing — explicit (closed before work), foreground (stays open), auto-background (closes at detach)
• adk/agent_tool: open gate forwards, closed gate drops, closed parent generator on gated path does not panic

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alpha/10@c8af907). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
adk/agent_tool.go	86.66%	2 Missing ⚠️
adk/backgroundtask/manager.go	95.83%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/10    #1128   +/-   ##
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  Coverage            ?   81.84%           
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  Files               ?      189           
  Lines               ?    30578           
  Branches            ?        0           
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  Hits                ?    25028           
  Misses              ?     3737           
  Partials            ?     1813           

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