Support sub-agent escalation event in ParallelAgent

[jinnigu]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Support sub-agent escalation event in ParallelAgent #5104

2. Or, if no issue exists, describe the change:

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.

Solution:
When any sub-agent emits an escalation event, the ParallelAgent now yields the event, cancels all remaining sub-agent tasks, and marks itself as complete in resumable mode. This works for both the Python 3.11+ (TaskGroup) and pre-3.11 code paths.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.

• All unit tests pass locally.

• test_run_async_short_circuits_other_agents_on_escalate_action — parameterized over is_resumable (True/False) x use_pre_3_11_merge (True/False) = 4 test variants. Verifies the slow agent is cancelled, post-escalation events are suppressed, correct event ordering, and proper end_of_agent state in resumable mode.

• test_resumable_parallel_agent_escalation_short_circuits_persisted_run — integration test using InMemoryRunner that verifies events are correctly persisted to the session store and that re-running a completed escalated invocation is a no-op.

• All 15 tests pass (12 in test_parallel_agent.py, 3 in test_resume_invocation.py), no regressions.

Manual End-to-End (E2E) Tests:

N/A — behavior is fully covered by unit and integration tests.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

• Java counterpart: google/adk-java#612
• LoopAgent already supports escalation; this brings ParallelAgent to parity.