parallelagent: escalate from a sub-agent does not stop the other branches #1784

When a direct sub-agent of a ParallelAgent yields an event with Actions.Escalate set (for example from exit_loop), the parallel agent keeps running the other branches to completion and keeps yielding their events (agent/workflowagents/parallelagent/agent.go, the results loop never checks Escalate).

adk-python stops and cancels the siblings after yielding that event (agents/parallel_agent.py, _asks_this_agent_to_exit), asserted by test_run_async_short_circuits_other_agents_on_escalate_action in tests/unittests/agents/test_parallel_agent.py.

Fix with regression tests: #1781

Written with the help of an agent (Claude Code, claude-opus-5-5).