validate: force the ci-serialisation baseline leg (not for merge)

.github/workflows/ci.yml

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+# Sandbox validation of CI_Toolkit PR #9: forces the ci-serialisation baseline leg.
+# Pinned to the fix branch. Not for merge.
+name: CI
+on:
+  pull_request:
+    branches: [develop]
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  ci-serialisation:
+    runs-on: windows-2025-vs2026
+    timeout-minutes: 90
+    steps:
+      - uses: BuroHappoldEngineeringSandbox/CI_Toolkit/.github/actions/ci-serialisation@validate/item4-integration
+        with:
+          app_id: ${{ secrets.BHOM_APP_ID }}
+          private_key: ${{ secrets.BHOM_APP_PRIVATE_KEY }}
+          base_ref: ${{ github.base_ref }}

XML_oM/Item4Fixture/UnserialisableFixture.cs

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+/*
+ * DELIBERATE SERIALISATION FIXTURE. NOT FOR MERGE.
+ *
+ * Authored to validate CI_Toolkit PR #9 (findings-register item 4). ci-serialisation only
+ * runs its baseline leg when the branch leg reports status == 'Error', and no sandbox repo
+ * currently produces a serialisation failure, so the baseline path had never executed under
+ * the fix. This type forces one.
+ *
+ * Why it fails: Helpers.ObjectTypesToTest() selects every non-abstract IObject whose every
+ * property sits in a BH.* or System.* namespace, so System.IntPtr passes selection. The BHoM
+ * serialiser has no BSON mapping for IntPtr, so ToJson() throws, Verify records an error, the
+ * json comes back empty and ObjectToFromJson returns TestStatus.Error for this type.
+ *
+ * The failure is subject-side and exists only on this branch, so the baseline leg should not
+ * see it and the comparator should report a regression. That is the intended outcome.
+ */
+
+using System;
+using System.Collections.Generic;
+
+using BH.oM.Base;
+
+namespace BH.oM.XML.Item4Fixture
+{
+    public class UnserialisableFixture : IBHoMObject
+    {
+        // The whole fixture: IntPtr has no BSON serialiser.
+        public virtual IntPtr Handle { get; set; } = IntPtr.Zero;
+
+        public virtual Guid BHoM_Guid { get; set; } = Guid.NewGuid();
+        public virtual Dictionary<string, object> CustomData { get; set; } = new Dictionary<string, object>();
+        public virtual string Name { get; set; } = "";
+        public virtual FragmentSet Fragments { get; set; } = new FragmentSet();
+        public virtual HashSet<string> Tags { get; set; } = new HashSet<string>();
+    }
+}