agent: unify listening and dialing with directed mutual measurements

Replace Probe with Agent across node runtime, Fleet contracts, backend state, API and storage. Report incoming sessions and outgoing link health through each Agent uplink, and retain directed link controls and retry.

Update protocol fixtures and end-to-end coverage for the unified model; accept requested cancellation during the Fleet timeout test cleanup.

backend/tests/MoShiro.Backend.Tests/UpdateTests.cs

@@ -51,11 +51,11 @@ public sealed class UpdateTests : IClassFixture<BackendFixture>
         await using AgentSimulator late = await _backend.ConnectAgentAsync();
         await BackendFixture.EventuallyAsync(() => Task.FromResult(late.UpdateOffers.ToArray().Any(o => o.Manifest == manifest)), because: "a node that connects later still gets the latest offer");
 
-        // 另一个组件（probe）的清单不会发给 Agent
-        using HttpResponseMessage probeRelease = await PublishAsync(Manifest("probe", seq));
-        Assert.Equal(HttpStatusCode.Created, probeRelease.StatusCode);
+        // 另一个组件（backend）的清单不会发给 Agent
+        using HttpResponseMessage backendRelease = await PublishAsync(Manifest("backend", seq));
+        Assert.Equal(HttpStatusCode.Created, backendRelease.StatusCode);
         await Task.Delay(300);
-        Assert.DoesNotContain(online.UpdateOffers.ToArray(), o => o.Manifest.Contains("\"component\":\"probe\"", StringComparison.Ordinal));
+        Assert.DoesNotContain(online.UpdateOffers.ToArray(), o => o.Manifest.Contains("\"component\":\"backend\"", StringComparison.Ordinal));
     }
 
     [Fact]