test: Campaign LA finish LA2 probe and idle gates

Prove idle play remains passive and live until cancellation, then converges through one truthful status teardown. Keep probe mode string-only so numeric enum aliases cannot expand the pinned v1 contract, and record the Windows/WSL gates.

tests/AcDream.Headless.Tests/HeadlessConfigurationLoaderTests.cs

@@ -237,6 +237,38 @@ public sealed class HeadlessConfigurationLoaderTests
         Assert.Null(session.Policy);
     }
 
+    /// <summary>
+    /// The pinned v1 contract has one named mode value: <c>"probe"</c>.
+    /// In particular, the enum's underlying numeric zero must not become an
+    /// accidental second spelling through an enum converter configured to
+    /// allow integers.
+    /// </summary>
+    [Theory]
+    [InlineData("\"play\"")]
+    [InlineData("0")]
+    public void SessionModeRejectsEveryValueOtherThanTheNamedProbeMode(
+        string modeJson)
+    {
+        using TemporaryConfiguration file = TemporaryConfiguration.Create(
+            $$"""
+            {
+              "version": 1,
+              "sessions": [
+                {
+                  "id": "unsupported-mode",
+                  "endpoint": { "host": "127.0.0.1", "port": 9000 },
+                  "account": "account",
+                  "mode": {{modeJson}},
+                  "credential": { "provider": "environment", "reference": "PROBE_PASSWORD" }
+                }
+              ]
+            }
+            """);
+
+        Assert.Throws<JsonException>(
+            () => HeadlessConfigurationLoader.Load(file.Path));
+    }
+
     [Fact]
     public void ProbeSessionDeclaringCharacterFailsLoadNamingTheField()
     {