feat(agents): let nested LoopAgents exit independently

[a2105z]

Summary

• Nested LoopAgents currently treat every escalate as global: an inner exit_loop() also kills every ancestor loop. That is the Nested LoopAgents will escalate all LoopAgents #2808 report (expect 25 inner steps, get 5).
• This implements @Jacksunwei's scoped-escalation design. escalate=True with no context stays root (today's behavior, including exit_loop). EscalationContext(type='parent') is consumed by the nearest LoopAgent, which records itself on handled_by so outer loops continue. exit_current_loop is the parent-scoped tool.
• Optional target_agent aims the exit at a named loop. ParallelAgent ignores already-consumed parent escalations so siblings of an inner loop keep running.

Test plan

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

uv run python -m pytest \
  tests/unittests/agents/test_loop_agent.py \
  tests/unittests/events/test_event_actions.py \
  tests/unittests/tools/test_exit_loop_tool.py \
  tests/unittests/agents/test_parallel_agent.py \
  tests/unittests/a2a/converters/test_to_adk.py::TestToAdk::test_peer_supplied_actions_cannot_mutate_caller_session \
  tests/unittests/a2a/converters/test_to_adk.py::TestToAdk::test_peer_settable_action_fields_are_exactly_inert \
  -q

57 loop/event/tool/parallel tests passed, plus the A2A peer-actions allow-list (escalation_context is not peer-settable).

Manual check: nested SequentialAgent → outer LoopAgent → inner LoopAgent with exit_current_loop should run inner×outer iterations; exit_loop still unwinds every loop.

Closes: #2808

@Jacksunwei @klateefa @hangfei @boyangsvl @wuliang229 @yeesian @xuanyang15 @steren @llalitkumarrr @GWeale @jhonratiu