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/server/format_auto_internal_test.go

@@ -0,0 +1,46 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"testing"
+)
+
+// TestFormatAutoVaryNextToOrigin requests an image URL with the format auto
+// through the server's routes and verifies that the answer carries
+// Vary: Accept next to the Vary: Origin the CORS middleware sends.
+func TestFormatAutoVaryNextToOrigin(t *testing.T) {
+	t.Parallel()
+
+	source := encodeTestPNG(t, 64, 48)
+
+	upstream := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.Header().Set("Content-Type", "image/png")
+			_, _ = w.Write(source)
+		}))
+	t.Cleanup(upstream.Close)
+
+	s, _, _ := startImageProxy(t, upstream)
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+		"/v1/image/"+upstreamHost+"/photo.png/32x24.auto", nil)
+	req.Header.Set("Origin", "https://app.example.com")
+	req.Header.Set("Accept", "image/webp")
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, req)
+
+	vary := rec.Header().Values("Vary")
+	t.Logf("status %d, Content-Type %s, Vary %v",
+		rec.Code, rec.Header().Get("Content-Type"), vary)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if want := []string{"Origin", "Accept"}; !slices.Equal(vary, want) {
+		t.Errorf("Vary = %v, want %v", vary, want)
+	}
+}