Test ETag, 304 and HEAD on /v1/e/

An image served through an encrypted URL must carry an ETag, a request
whose If-None-Match is that ETag must get 304 with no body, HEAD must get
200 with the headers and no body, and the server must route HEAD on
/v1/e/ to its handler instead of answering 405.

Model: opus-5-5

internal/server/routes_internal_test.go

@@ -0,0 +1,27 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestEncryptedImageRouteAnswersHEAD verifies that HEAD on the encrypted image
+// route reaches its handler, as GET does, instead of being answered 405 Method
+// Not Allowed. The handler refuses a token it cannot decrypt with 400, so that
+// status shows the request got through.
+func TestEncryptedImageRouteAnswersHEAD(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodHead, encryptedImagePath, nil))
+	t.Logf("status %d", rec.Code)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Errorf("status = %d, want %d from the encrypted image handler",
+			rec.Code, http.StatusBadRequest)
+	}
+}