Add failing tests for the format auto

Tests for #88, before the change:
the format chosen for each Accept header (AVIF-capable, WebP-only,
neither, wildcard only, absent, q=0 on AVIF, malformed), both image
routes serving an auto URL in the chosen format with Vary: Accept, also
on HEAD and 304, the signature covering auto rather than the chosen
format, each chosen format cached apart, and Vary: Accept next to the
CORS middleware's Vary: Origin. They do not compile yet: FormatAuto and
formatForAccept do not exist.

Model: opus-5-5

internal/imgcache/urlparser_internal_test.go

@@ -111,6 +111,21 @@ func TestParseImageURL(t *testing.T) {
 	}
 }
 
+// TestParseImageURL_AutoFormat verifies that the format auto in an image URL
+// parses as FormatAuto.
+func TestParseImageURL_AutoFormat(t *testing.T) {
+	t.Parallel()
+
+	got, err := ParseImageURL("/v1/image/example.com/photo.jpg/200x200.auto")
+	if err != nil {
+		t.Fatalf("ParseImageURL() error = %v", err)
+	}
+
+	if got.Format != FormatAuto {
+		t.Errorf("Format = %q, want %q", got.Format, FormatAuto)
+	}
+}
+
 func TestParseImageURL_Errors(t *testing.T) {
 	t.Parallel()