Fix update-api-unshipped to handle dropped preview APIs

Agent-Logs-Url: https://github.com/petesramek/polyline-algorithm-csharp/sessions/b29b300b-90b6-496a-a4d6-a2f7e600bc3a

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>

Author: Copilot

.github/workflows/release.yml

@@ -171,8 +171,10 @@ jobs:
 
   update-api-unshipped:
     name: 'Update PublicAPI.Unshipped.txt'
-    needs: [workflow-variables, build, validate-release]
-    if: ${{ needs.workflow-variables.outputs.is-preview == 'true' }}
+    needs: [workflow-variables, validate-release]
+    # Run even when build fails — the build may fail solely because a dropped preview API is still
+    # listed in Unshipped.txt (RS0017). This job fixes exactly that, so it must not be gated on build.
+    if: ${{ needs.workflow-variables.outputs.is-preview == 'true' && needs.validate-release.result == 'success' }}
     runs-on: ubuntu-latest
     steps:
       - name: 'Checkout ${{ github.ref }}'
@@ -183,9 +185,25 @@ jobs:
         with:
           dotnet-version: ${{ env.dotnet-sdk-version }}
 
-      - name: 'Add missing API declarations to PublicAPI.Unshipped.txt'
+      - name: 'Snapshot PublicAPI.Shipped.txt'
+        shell: bash
+        run: cp src/PolylineAlgorithm/PublicAPI.Shipped.txt src/PolylineAlgorithm/PublicAPI.Shipped.txt.bak
+
+      - name: 'Sync PublicAPI.Unshipped.txt (add new + remove dropped preview APIs)'
+        shell: bash
+        run: dotnet format analyzers src/PolylineAlgorithm/PolylineAlgorithm.csproj --diagnostics RS0016 RS0017
+
+      - name: 'Guard against accidental shipped API removal'
         shell: bash
-        run: dotnet format analyzers src/PolylineAlgorithm/PolylineAlgorithm.csproj --diagnostics RS0016
+        run: |
+          SHIPPED="src/PolylineAlgorithm/PublicAPI.Shipped.txt"
+          if ! diff -q "$SHIPPED" "$SHIPPED.bak" > /dev/null 2>&1; then
+            echo "::error::Breaking change detected — a shipped API was removed from PublicAPI.Shipped.txt."
+            echo "::error::This requires a major version bump. Reverting the unintended change."
+            cp "$SHIPPED.bak" "$SHIPPED"
+            exit 1
+          fi
+          rm -f "$SHIPPED.bak"
 
       - name: 'Configure git identity'
         uses: './.github/actions/git/configure-identity'
@@ -195,7 +213,7 @@ jobs:
         run: |
           git add src/PolylineAlgorithm/PublicAPI.Unshipped.txt
           git diff --staged --quiet || (
-            git commit -m "Update PublicAPI.Unshipped.txt with new API declarations" &&
+            git commit -m "Sync PublicAPI.Unshipped.txt (add new, remove dropped preview APIs)" &&
             git pull --rebase origin ${{ github.ref_name }} &&
             git push
           )