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/shared/MoShiro.Measurement/Platform/ReportingSection.cs

@@ -2,7 +2,7 @@ using System.Text.Json.Serialization;
 
 namespace MoShiro.Measurement;
 
-/// <summary>节点本地配置里的 <c>reporting</c> 段（面板无法修改）：Agent 和 Probe 共用。</summary>
+/// <summary>节点本地配置里的 <c>reporting</c> 段（面板无法修改）：拨号方和监听方共用。</summary>
 public sealed class ReportingSection
 {
     /// <summary>上报主机资源占用（CPU、内存、磁盘、流量、运行时长，只有汇总数字，没有型号和 IP）。设为 false 就一个字也不上报。</summary>