Add tests for unsupported output format errors

Tests verify that WebP and AVIF encoding requests return
ErrUnsupportedOutputFormat instead of silently falling back
to a different format.

internal/imgcache/processor_test.go

@@ -3,6 +3,7 @@ package imgcache
 import (
 	"bytes"
 	"context"
+	"errors"
 	"image"
 	"image/color"
 	"image/jpeg"
@@ -311,3 +312,49 @@ func TestImageProcessor_AcceptsMaxDimensionInput(t *testing.T) {
 	}
 	defer result.Content.Close()
 }
+
+func TestImageProcessor_RejectsUnsupportedOutputFormat_WebP(t *testing.T) {
+	proc := NewImageProcessor()
+	ctx := context.Background()
+
+	input := createTestJPEG(t, 200, 150)
+
+	req := &ImageRequest{
+		Size:    Size{Width: 100, Height: 75},
+		Format:  FormatWebP,
+		Quality: 85,
+		FitMode: FitCover,
+	}
+
+	_, err := proc.Process(ctx, bytes.NewReader(input), req)
+	if err == nil {
+		t.Fatal("Process() should return error for unsupported WebP encoding")
+	}
+
+	if !errors.Is(err, ErrUnsupportedOutputFormat) {
+		t.Errorf("Process() error = %v, want ErrUnsupportedOutputFormat", err)
+	}
+}
+
+func TestImageProcessor_RejectsUnsupportedOutputFormat_AVIF(t *testing.T) {
+	proc := NewImageProcessor()
+	ctx := context.Background()
+
+	input := createTestJPEG(t, 200, 150)
+
+	req := &ImageRequest{
+		Size:    Size{Width: 100, Height: 75},
+		Format:  FormatAVIF,
+		Quality: 85,
+		FitMode: FitCover,
+	}
+
+	_, err := proc.Process(ctx, bytes.NewReader(input), req)
+	if err == nil {
+		t.Fatal("Process() should return error for unsupported AVIF encoding")
+	}
+
+	if !errors.Is(err, ErrUnsupportedOutputFormat) {
+		t.Errorf("Process() error = %v, want ErrUnsupportedOutputFormat", err)
+	}
+}