Surface agent interrupts as stream events

[Copilot]

Agent HITL interrupts were only available on AgentResult, leaving streaming consumers unable to distinguish a paused invocation from a generic error. This adds an explicit interrupt event path so SSE/EventQueue clients can collect pending interrupt IDs and resume the agent.

• Stream event contract

  • Added EventType.INTERRUPT with wire value "interrupt".
  • Documented the new event in streaming event lists.

• EventPublisher behavior

  • Emits one interrupt StreamEvent per pending interrupt when AfterInvocationEvent.result.stop_reason == "interrupt".
  • Suppresses complete for interrupted invocations, since the session is paused rather than finished.

• Payload shape

  StreamEvent(
      type=EventType.INTERRUPT,
      agent_name="agent",
      data={
          "interrupt_id": interrupt.id,
          "name": interrupt.name,
          "reason": interrupt.reason,
      },
  )

• Coverage

  • Added focused tests for interrupt event emission, complete suppression, and enum coverage.

[galuszkm]

lgtm