web: reset links to the default mode, and keep CN agents dial-only by default [CI]

POST /api/admin/links/reset clears every manual link setting and turns automatic linking on. In the default rules a listening node in CN ranks below a listening node elsewhere, so the CN side dials; the links are pushed again when a node's country is first resolved or changes.

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

@@ -1,5 +1,7 @@
 using System.Net;
 using System.Text.Json.Nodes;
+using Fleet.Server;
+using MoShiro.Backend.Application;
 using MoShiro.TestSupport;
 
 namespace MoShiro.Backend.Tests;
@@ -60,6 +62,56 @@ public sealed class LinkTests : IClassFixture<BackendFixture>
         Assert.True((await LinkAsync(a, b))!["linked"]!.GetValue<bool>());
     }
 
+    private static AgentRecord Node(string id, bool listens) =>
+        new(new Fleet.Core.NodeId(id), id, NodeStatus.Active, [], [], "", 0, [], [], true, [], null, null, 0, 0, null, 0, null, 0, Listens: listens);
+
+    [Fact]
+    public void DefaultDialer_NonListeningDials_ThenDomesticDialsForeign_ThenSmallerId()
+    {
+        AgentRecord home = Node("agt_a", listens: false);
+        AgentRecord cn = Node("agt_b", listens: true);
+        AgentRecord abroad = Node("agt_c", listens: true);
+        AgentRecord abroad2 = Node("agt_d", listens: true);
+        var snapshot = new LinkSnapshot(true, [], ["agt_b"]);
+
+        Assert.True(snapshot.IsLinked(home, cn));            // 不监听的拨监听的
+        Assert.False(snapshot.IsLinked(cn, home));
+        Assert.True(snapshot.IsLinked(cn, abroad));          // 国内的拨国外的，哪怕它的 id 更小
+        Assert.False(snapshot.IsLinked(abroad, cn));
+        Assert.True(snapshot.IsLinked(abroad, abroad2));     // 一样的按 id，小的拨
+        Assert.False(snapshot.IsLinked(abroad2, abroad));
+        Assert.True(snapshot.IsLinked(home, cn));            // 只有国内的在监听时，国内的也得当监听方
+
+        // 手动指定的方向不受国家影响：让国外的拨国内的
+        var manual = new LinkSnapshot(true, new() { [("agt_c", "agt_b")] = true }, ["agt_b"]);
+        Assert.True(manual.IsLinked(abroad, cn));
+        Assert.False(manual.IsLinked(cn, abroad));
+    }
+
+    [Fact]
+    public async Task Links_ResetDropsManualSettingsAndTurnsAutomaticLinkingOn()
+    {
+        await using AgentSimulator first = await _backend.ConnectAgentAsync();
+        await using AgentSimulator second = await _backend.ConnectAgentAsync();
+        string[] ids = [.. new[] { first.NodeId.Value, second.NodeId.Value }.Order(StringComparer.Ordinal)];
+        await SetLinkAsync(ids[1], ids[0], true); // 手动让大 id 的拨小 id 的
+        await SetAutoLinkAsync(false);
+        try
+        {
+            using HttpResponseMessage reset = await _backend.Admin.PostAsync("/api/admin/links/reset", BackendFixture.Json(new JsonObject()));
+            reset.EnsureSuccessStatusCode();
+            JsonNode matrix = JsonNode.Parse(await reset.Content.ReadAsStringAsync())!;
+            Assert.True(matrix["auto_link"]!.GetValue<bool>());
+            JsonNode link = (await LinkAsync(ids[0], ids[1]))!; // 回到默认方向：小 id 拨大 id
+            Assert.True(link["linked"]!.GetValue<bool>());
+            Assert.Null(link["override"]);
+        }
+        finally
+        {
+            await SetAutoLinkAsync(true);
+        }
+    }
+
     [Fact]
     public async Task Links_FollowTheAutomaticSwitch_AndManualChoicesAlwaysWin()
     {