✅ Guard the my/routes manifest snake_case wire contract

The acceptance test deserializes the response into the C# record, which is
casing-agnostic, so it would not catch a regression of the emitted field names.
Add a serialization test asserting RouteManifestEntry emits snake_case even under
a camelCase policy (as the Monitoring instance uses), pinning the contract that
[JsonPropertyName] enforces.

Author: dvdstelt

src/ServiceControl.Infrastructure.Tests/Auth/RouteManifestEntrySerializationTests.cs

@@ -0,0 +1,27 @@
+#nullable enable
+namespace ServiceControl.Infrastructure.Tests.Auth;
+
+using System.Text.Json;
+using NUnit.Framework;
+using ServiceControl.Infrastructure.Auth;
+
+[TestFixture]
+class RouteManifestEntrySerializationTests
+{
+    // The my/routes manifest must have ONE wire shape across instances. The Primary instance
+    // serializes snake_case and the Monitoring instance camelCase, so RouteManifestEntry pins its
+    // field names with [JsonPropertyName]. This test guards that contract: even under a camelCase
+    // policy (as on the Monitoring host) the emitted names stay snake_case, so a client merging both
+    // instances never silently drops the differently-cased entries.
+    [Test]
+    public void Emits_snake_case_field_names_even_under_a_camelCase_policy()
+    {
+        var json = JsonSerializer.Serialize(
+            new RouteManifestEntry("GET", "/api/errors"),
+            new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase });
+
+        Assert.That(json, Does.Contain("\"method\""));
+        Assert.That(json, Does.Contain("\"url_template\""));
+        Assert.That(json, Does.Not.Contain("urlTemplate"));
+    }
+}