feat: add audited one-shot prepare gate

tests/MBN_STOCK_WEBVIEW.LegacyBridge.Tests/LegacyPlayoutBridgeTests.cs

@@ -8,6 +8,47 @@ namespace MBN_STOCK_WEBVIEW.LegacyBridge.Tests;
 
 public sealed class LegacyPlayoutBridgeTests
 {
+    private const string GateACapability =
+        "0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF";
+
+    [Fact]
+    public void GateAPrepare_ParsesOnlyExactUppercaseCapabilityShape()
+    {
+        Assert.True(LegacyBridgeProtocol.TryParseIntent(
+            "{\"type\":\"gate-a-prepare\",\"payload\":{\"capability\":\"" +
+                GateACapability + "\"}}",
+            out var intent,
+            out var error), error);
+
+        Assert.Equal(
+            GateACapability,
+            Assert.IsType<LegacyGateAPrepareIntent>(intent).Capability);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef")]
+    [InlineData("0123456789ABCDEF")]
+    [InlineData("G123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF")]
+    public void GateAPrepare_RejectsInvalidCapabilities(string capability)
+    {
+        Assert.False(LegacyBridgeProtocol.TryParseIntent(
+            "{\"type\":\"gate-a-prepare\",\"payload\":{\"capability\":\"" +
+                capability + "\"}}",
+            out _,
+            out _));
+    }
+
+    [Fact]
+    public void GateAPrepare_RejectsExtraProperties()
+    {
+        Assert.False(LegacyBridgeProtocol.TryParseIntent(
+            "{\"type\":\"gate-a-prepare\",\"payload\":{\"capability\":\"" +
+                GateACapability + "\",\"retry\":false}}",
+            out _,
+            out _));
+    }
+
     [Theory]
     [InlineData("prepare-playout", LegacyOperatorPlayoutCommand.Prepare)]
     [InlineData("take-in", LegacyOperatorPlayoutCommand.TakeIn)]