Add background-color paint-call test coverage

No test anywhere verified background-color actually paints a fill,
despite the paint code (CssBox.PaintBackground) already working -
only CSS-OM parsing (BackgroundPropertyTests.cs) was covered.
Verifies fill geometry, the transparent/default (also transparent)
no-op cases, and paint order relative to border/outline per CSS2.1
Appendix E.

Author: jhaygood86

Source/Test/HtmlRenderer.IntegrationTest/Painting/BackgroundPaintIntegrationTests.cs

@@ -0,0 +1,77 @@
+using System.Linq;
+using HtmlRenderer.IntegrationTest.TestSupport;
+using TheArtOfDev.HtmlRenderer.Adapters.Entities;
+
+namespace HtmlRenderer.IntegrationTest.Painting;
+
+/// <summary>
+/// CSS 2.1 §14.2.1: <c>background-color</c> paints a solid fill behind a box's content, covering the box's
+/// padding + content area (border-box minus the border itself). No dedicated background paint-call test
+/// existed anywhere in this repo before this - only CSS-OM parsing (<c>BackgroundPropertyTests.cs</c>).
+/// </summary>
+[DoNotParallelize]
+[TestClass]
+public sealed class BackgroundPaintIntegrationTests
+{
+    [TestMethod]
+    public void BackgroundColor_PaintsASolidFillRect()
+    {
+        var (root, container) = PaintHarness.Layout(PaintHarness.Wrap(
+            "<div id='b' style='width:100px;height:50px;background-color:rgb(10,20,30)'>x</div>"));
+        var div = PaintHarness.FindById(root, "b")!;
+
+        var g = PaintHarness.PaintBox(container, div);
+        var fills = g.Log.OfType<RecordingGraphics.DrawRectCall>()
+            .Where(r => r.Color == RColor.FromArgb(10, 20, 30)).ToList();
+
+        Assert.AreEqual(1, fills.Count);
+        Assert.AreEqual(100, fills[0].Width, 0.1);
+        Assert.AreEqual(50, fills[0].Height, 0.1);
+    }
+
+    [TestMethod]
+    public void BackgroundColorTransparent_PaintsNothing()
+    {
+        var (root, container) = PaintHarness.Layout(PaintHarness.Wrap(
+            "<div id='b' style='width:100px;height:50px;background-color:transparent'>x</div>"));
+        var div = PaintHarness.FindById(root, "b")!;
+
+        var g = PaintHarness.PaintBox(container, div);
+
+        Assert.IsFalse(g.Log.OfType<RecordingGraphics.DrawRectCall>().Any());
+    }
+
+    [TestMethod]
+    public void BackgroundColorDefault_PaintsNothing()
+    {
+        // The initial value of background-color is "transparent" - a box with no background-color set at
+        // all must not paint a fill either.
+        var (root, container) = PaintHarness.Layout(PaintHarness.Wrap(
+            "<div id='b' style='width:100px;height:50px;'>x</div>"));
+        var div = PaintHarness.FindById(root, "b")!;
+
+        var g = PaintHarness.PaintBox(container, div);
+
+        Assert.IsFalse(g.Log.OfType<RecordingGraphics.DrawRectCall>().Any());
+    }
+
+    [TestMethod]
+    public void BackgroundColor_PaintsBeforeBorderAndOutline()
+    {
+        // CSS 2.1 Appendix E: background paints before border/outline for the same box.
+        var (root, container) = PaintHarness.Layout(PaintHarness.Wrap(
+            "<div id='b' style='width:100px;height:50px;background-color:rgb(10,20,30);"
+            + "border:3px solid rgb(1,2,3);outline:5px solid rgb(4,5,6)'>x</div>"));
+        var div = PaintHarness.FindById(root, "b")!;
+
+        var g = PaintHarness.PaintBox(container, div);
+
+        var backgroundIndex = g.Log.FindIndex(c => c is RecordingGraphics.DrawRectCall r && r.Color == RColor.FromArgb(10, 20, 30));
+        var borderIndex = g.Log.FindIndex(c => c is RecordingGraphics.DrawLineCall l && l.Color == RColor.FromArgb(1, 2, 3));
+        var outlineIndex = g.Log.FindIndex(c => c is RecordingGraphics.DrawRectCall r && r.Color == RColor.FromArgb(4, 5, 6));
+
+        Assert.IsTrue(backgroundIndex >= 0 && borderIndex >= 0 && outlineIndex >= 0);
+        Assert.IsTrue(backgroundIndex < borderIndex, "background must paint before the border");
+        Assert.IsTrue(borderIndex < outlineIndex, "border must paint before the outline");
+    }
+}