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/fleet/src/Fleet.Rpc/PeerMethods.cs

@@ -3,7 +3,7 @@ using System.Text.Json.Serialization;
 namespace Fleet.Rpc;
 
 /// <summary>
-/// 节点间点对点会话的握手（Probe → Agent，TLS 帧）。除了签名认证，双方各生成一个临时 ECDH 密钥，
+/// 节点间点对点会话的握手（拨号方 → 监听方，TLS 帧）。除了签名认证，双方各生成一个临时 ECDH 密钥，
 /// 经过 HKDF 派生出数据面使用的 <c>session_id</c> 与 <c>session_key</c>（所以不需要单独的“下发密钥”方法）。
 /// </summary>
 public static class PeerMethods