[PictureLoader] Improve image negotiation, cache recovery and redirects (#7703)

* Fix picture-loader cache formats, redirects and offline queue handling

* Format picture-loader changes using project style

* Address review: name decoded image fields, test cancelled requests

Use designated initializers for DecodedImage so a field reorder cannot
silently swap the image and its error string.

Extract the cancelled-request sweep shared by the dispatcher and the
single-request path into takeCancelledRequests(), and cover it: one status
retirement per vanished work item, in queue order, with live entries kept.

---------

Co-authored-by: Lukas Brübach <44814898+BruebachL@users.noreply.github.com>

tests/image_request_queue_test.cpp

@@ -0,0 +1,85 @@
+#include "../cockatrice/src/interface/card_picture_loader/card_picture_loader_request_queue.h"
+
+#include <QObject>
+#include <QPointer>
+#include <QQueue>
+#include <QUrl>
+#include <gtest/gtest.h>
+
+namespace
+{
+using TestRequest = QPair<QUrl, QPointer<QObject>>;
+
+QUrl url(int index)
+{
+    return QUrl(QStringLiteral("https://cards.example/%1").arg(index));
+}
+
+TestRequest live(int index, QObject &owner)
+{
+    return qMakePair(url(index), QPointer<QObject>(&owner));
+}
+
+TestRequest cancelled(int index)
+{
+    return qMakePair(url(index), QPointer<QObject>());
+}
+
+QList<QUrl> queuedUrls(const QQueue<TestRequest> &queue)
+{
+    QList<QUrl> urls;
+    for (const auto &entry : queue) {
+        urls.append(entry.first);
+    }
+    return urls;
+}
+} // namespace
+
+TEST(PictureLoaderRequestQueue, EmptyQueueHasNothingToRetire)
+{
+    QQueue<TestRequest> queue;
+    EXPECT_TRUE(takeCancelledRequests(queue).isEmpty());
+    EXPECT_TRUE(queue.isEmpty());
+}
+
+TEST(PictureLoaderRequestQueue, LiveRequestsStayQueuedInOrder)
+{
+    QObject first;
+    QObject second;
+    QQueue<TestRequest> queue;
+    queue.enqueue(live(1, first));
+    queue.enqueue(live(2, second));
+
+    EXPECT_TRUE(takeCancelledRequests(queue).isEmpty());
+    EXPECT_EQ(queuedUrls(queue), (QList<QUrl>{url(1), url(2)}));
+}
+
+TEST(PictureLoaderRequestQueue, CancelledRequestsAreRetiredInQueueOrder)
+{
+    QObject owner;
+    QQueue<TestRequest> queue;
+    queue.enqueue(live(1, owner));
+    queue.enqueue(cancelled(2));
+    queue.enqueue(live(3, owner));
+    queue.enqueue(cancelled(4));
+    queue.enqueue(cancelled(5));
+
+    // Exactly one status entry is retired per vanished work item, in queue order.
+    EXPECT_EQ(takeCancelledRequests(queue), (QList<QUrl>{url(2), url(4), url(5)}));
+    // The surviving requests keep their relative order.
+    EXPECT_EQ(queuedUrls(queue), (QList<QUrl>{url(1), url(3)}));
+
+    // Nothing is retired twice on a later sweep.
+    EXPECT_TRUE(takeCancelledRequests(queue).isEmpty());
+    EXPECT_EQ(queuedUrls(queue), (QList<QUrl>{url(1), url(3)}));
+}
+
+TEST(PictureLoaderRequestQueue, AllCancelledEmptiesTheQueue)
+{
+    QQueue<TestRequest> queue;
+    queue.enqueue(cancelled(1));
+    queue.enqueue(cancelled(2));
+
+    EXPECT_EQ(takeCancelledRequests(queue), (QList<QUrl>{url(1), url(2)}));
+    EXPECT_TRUE(queue.isEmpty());
+}