Support sub-agent escalation event in ParallelAgent

[jinnigu]

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Is your feature request related to a specific problem?

ParallelAgent does not short-circuit when a sub-agent emits an escalation event (actions.escalate = True). Other sub-agents continue running to completion unnecessarily.

This is the Python counterpart of google/adk-java#561, resolved in google/adk-java#612.

Describe the Solution You'd Like

When any sub-agent emits an event with actions.escalate = True, the ParallelAgent should yield the escalation event, immediately cancel all remaining sub-agent tasks, and mark itself as complete in resumable mode. This should work for both the Python 3.11+ and pre-3.11 code paths.

Impact on your work

Needed for feature parity between adk-java and adk-python. LoopAgent already supports escalation; ParallelAgent should too.

Willingness to contribute

Yes

[klateefa]

@jinnigu Thank you for submitting this. It’s currently under review by our team, and I’ll keep you posted on any updates. We appreciate your input!