feat(runtime): track node lifecycle and QUIC link state

backend/shared/tests/MoShiro.Protocol.Tests/NodeRuntimeStatusTests.cs

@@ -0,0 +1,17 @@
+using System.Text.Json;
+
+namespace MoShiro.Protocol.Tests;
+
+public sealed class NodeRuntimeStatusTests
+{
+    [Fact]
+    public void HeartbeatStatus_UsesStableSnakeCaseEnumValues()
+    {
+        var heartbeat = new AgentHeartbeatExtra { RuntimeStatus = NodeRuntimeStatus.ProbePaused };
+        string json = JsonSerializer.Serialize(heartbeat, MoShiroJsonContext.Default.AgentHeartbeatExtra);
+
+        Assert.Contains("\"runtime_status\":\"probe_paused\"", json, StringComparison.Ordinal);
+        Assert.Equal(NodeRuntimeStatus.ProbePaused,
+            JsonSerializer.Deserialize(json, MoShiroJsonContext.Default.AgentHeartbeatExtra)!.RuntimeStatus);
+    }
+}