refactor: replace boolean endInvocation flag with atomic.Bool to enable cross-context state propagation

[hanorik]

Fixes an issue where EndInvocation() called inside sub-agent callbacks or child nodes did not propagate to parent/sibling contexts because EndInvocation was stored as a value-copied bool.

Changes

• Shared State: Replaced EndInvocation bool with *atomic.Bool across InvocationContext implementations (agent and internal/context).
• Propagation: Added EndInvocationPtr() *atomic.Bool so derived child contexts (WithContext, WithICDelta, parallelagent, workflow.AgentNode) share the same underlying atomic flag.
• Orchestration: Updated sequentialagent and loopagent to check ctx.Ended() and stop execution when a sub-agent ends the invocation.

Testing Plan

• Added unit tests in sequentialagent, agent, and internal/context packages verifying that EndInvocation() propagates across child/sub-agent contexts.
• Ran go test -race ./....

[karolpiotrowicz]

Thanks for this — the value-copy propagation bug is real, and the red→green tests (TestSubAgentCallbackEndInvocationPropagated, TestSequentialAgent_SubAgentEndInvocationPropagated) capture it well. I reproduced it locally: both new tests fail on main and pass with this change, the full go test -race ./... suite is green (one unrelated remoteagent timing flake), and there's no public API change. Keeping EndInvocationPtr() off the public interface via a type assertion is a nice touch.

I left a few inline notes. The main things I'd like to see before merge:

• Live path isn't covered — sequentialAgent.RunLive doesn't have the Ended() short-circuit that Run now has (inline).
• ParallelAgent semantics need a deliberate decision — sharing one flag across siblings changes isolation behavior (inline).
• initEndInvocation isn't thread-safe — worth eager-initializing (inline).
• Test coverage — loopagent, parallelagent (shared atomic), and workflow/agent_node.go are changed but untested; could you add cases for those?
• Rebase on main — the branch predates the runner.NewInMemory addition, so an API diff against current main looks misleadingly like a removal; a rebase clears it. (It also sets up the small follow-up that #370 describes.)

Overall the core mechanism and the sequential propagation are solid — nice work.

[hanorik]

Also added missing test coverage.

[wolo-lab]

The fix looks good.

There's one thing worrying me. adk-python decided explicitly not to fix it: google/adk-python#1452.

Submitting this fix will introduce the difference in behavior between adks. Let me sync with them first.