feat(relay): route mid-turn /stop over relay interrupt channel

RelayAdapter.on_interrupt(session_key, chat_id) bridges a connector-delivered
mid-turn /stop into the existing interrupt_session_activity path, setting the
per-session _active_sessions Event and clearing typing — cancelling exactly the
targeted session's turn without touching siblings (mirrors test_stop_thread_
sibling isolation). Transport.send_interrupt carries the gateway-side egress to
the connector for socket-owner routing.

Phase 1, Task 1.4 of the gateway-relay plan.

tests/gateway/relay/test_relay_interrupt.py

@@ -0,0 +1,69 @@
+"""Relay /stop interrupt routing (relay Phase 1, Task 1.4).
+
+Proves a connector-delivered mid-turn interrupt reaches the existing per-session
+interrupt mechanism and cancels exactly the targeted session_key's turn — never
+a sibling's. Mirrors the isolation discipline of test_stop_thread_sibling.py.
+"""
+
+from __future__ import annotations
+
+import asyncio
+
+import pytest
+
+from gateway.config import PlatformConfig
+from gateway.relay.adapter import RelayAdapter
+from gateway.relay.descriptor import CONTRACT_VERSION, CapabilityDescriptor
+
+from tests.gateway.relay.stub_connector import StubConnector
+
+
+def _desc() -> CapabilityDescriptor:
+    return CapabilityDescriptor(
+        contract_version=CONTRACT_VERSION,
+        platform="discord",
+        label="Discord",
+        max_message_length=2000,
+        supports_draft_streaming=False,
+        supports_edit=True,
+        supports_threads=True,
+        markdown_dialect="discord",
+        len_unit="chars",
+    )
+
+
+@pytest.fixture
+def adapter():
+    return RelayAdapter(PlatformConfig(), _desc(), transport=StubConnector(_desc()))
+
+
+@pytest.mark.asyncio
+async def test_interrupt_sets_only_target_session_event(adapter):
+    key_a = "agent:main:discord:group:chanA:userX"
+    key_b = "agent:main:discord:group:chanB:userY"
+    ev_a = asyncio.Event()
+    ev_b = asyncio.Event()
+    adapter._active_sessions[key_a] = ev_a
+    adapter._active_sessions[key_b] = ev_b
+
+    await adapter.on_interrupt(key_a, chat_id="chanA")
+
+    assert ev_a.is_set() is True, "target session's interrupt Event must be set"
+    assert ev_b.is_set() is False, "sibling session must be untouched"
+
+
+@pytest.mark.asyncio
+async def test_interrupt_unknown_session_is_noop(adapter):
+    # No active session for this key — must not raise.
+    await adapter.on_interrupt("agent:main:discord:group:nope:userZ", chat_id="nope")
+
+
+@pytest.mark.asyncio
+async def test_outbound_interrupt_reaches_connector(adapter):
+    """The gateway-side /stop egress: send_interrupt is carried to the connector
+    so it can forward down the socket owning the session_key."""
+    stub = adapter._transport
+    await stub.send_interrupt("agent:main:discord:group:chanA:userX", reason="stop")
+    assert stub.interrupts == [
+        {"session_key": "agent:main:discord:group:chanA:userX", "reason": "stop"}
+    ]