Handle empty featured_image from Columbia feed

Products with no featured image return featured_image: [] (empty array) rather
than an object or null, which failed to bind to a single ColumbiaImage and
aborted the whole sync. Adds ColumbiaImageJsonConverter that reads the object
when present and yields null for any non-object form ([], false, ""), and drops
the unused GalleryImages property (we only use featured_image) to remove the
same risk. Regression tests cover both the empty-array and object cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/PowderCoating.UnitTests/ColumbiaCatalogMapperTests.cs

@@ -230,6 +230,37 @@ public class ColumbiaCatalogMapperTests
 
     // ── End-to-end mapping invariants ─────────────────────────────────────
 
+    // ── Tolerant image deserialization (WordPress returns [] / false when empty) ──
+
+    private static readonly JsonSerializerOptions ClientJsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
+        PropertyNameCaseInsensitive = true,
+        Converters = { new ColumbiaImageJsonConverter() },
+    };
+
+    [Fact]
+    public void Deserialize_FeaturedImageEmptyArray_YieldsNullImage()
+    {
+        // WordPress returns featured_image: [] for products with no image.
+        var json = """{ "sku": "X1", "name": "No Image", "featured_image": [] }""";
+        var product = JsonSerializer.Deserialize<ColumbiaProduct>(json, ClientJsonOptions);
+
+        Assert.NotNull(product);
+        Assert.Null(product!.FeaturedImage);
+        Assert.Null(ColumbiaCatalogMapper.Map(product).ImageUrl);
+    }
+
+    [Fact]
+    public void Deserialize_FeaturedImageObject_ParsesSrc()
+    {
+        var json = """{ "sku": "X1", "name": "Has Image", "featured_image": { "id": 5, "src": "https://x/img.png", "name": "i", "alt": "" } }""";
+        var product = JsonSerializer.Deserialize<ColumbiaProduct>(json, ClientJsonOptions);
+
+        Assert.Equal("https://x/img.png", product!.FeaturedImage!.Src);
+        Assert.Equal("https://x/img.png", ColumbiaCatalogMapper.Map(product).ImageUrl);
+    }
+
     [Fact]
     public void Map_AlwaysStampsSource_AndLeavesEnrichmentFieldsNull()
     {