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/src/MoShiro.Backend.Application/Tests/Plans.cs

@@ -7,7 +7,7 @@ namespace MoShiro.Backend.Application;
 public sealed record PhaseSpec(string Kind, IReadOnlyDictionary<string, JsonElement> Overrides, int DurationSeconds);
 
 /// <summary>
-/// 测试方案：业务层的概念，只在 Backend。方案只是若干采集原语（Kind）阶段的编排，采集层（Agent、Probe、Measurement）不认识它。
+/// 测试方案：业务层的概念，只在 Backend。方案只是若干采集原语（Kind）阶段的编排，采集层（Agent、Measurement）不认识它。
 /// 测试方案：多个阶段的 once 任务组合（例如 hy2_ladder 在 10、25、50、100 Mbps 下各跑 10 秒）。</summary>
 public sealed record PlanSpec(string Id, string Title, string Description, IReadOnlyList<PhaseSpec> Phases);