tests: bring the slow mesh and fixed-rate cases in line with one line per pair and adaptive probing

The mesh test expects the single line of a pair, dialed by the smaller id. Tests that send at a fixed rate pin adaptive to false, so the admission limit follows that rate.

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

@@ -119,6 +119,7 @@ public sealed class AgentHarness : IAsyncDisposable
             bool enabled = kind != "connect";
             if (rateHz is { } hz && kind != "connect")
             {
+                parameters["adaptive"] = false; // 固定频率：用例按这个速率发包，限速也按它算
                 parameters["rate_hz"] = hz;
             }