Preserve installed card databases when Oracle refresh fails (#7739)

* Preserve installed card databases when Oracle refresh fails

* Fix the Oracle refresh verdict and cover the parser write path

A background refresh that already installed cards.xml exited 1 when the
token save afterwards failed, so the client kept its old in-memory
database and reported the update as failed. Return the same verdict
main() already computes: a reject after the database was written is a
successful update.

Cover the writer Oracle actually uses: add a short-write death test for
CockatriceXml4Parser::saveToFile, which reverted to QFile would truncate
an installed database without any test noticing.

Route a failed background token download through reject() instead of a
modal box on a hidden wizard, which left the client waiting with no
timeout and blocked later updates. Test the first non-space byte in
place rather than copying the payload, and report an import refusal with
the importer's own message.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

oracle/src/database_writer.cpp

@@ -0,0 +1,52 @@
+#include "database_writer.h"
+
+#include <QCoreApplication>
+#include <QIODevice>
+#include <QSaveFile>
+#include <QXmlStreamReader>
+
+namespace OracleDatabase
+{
+QString validateXml(const QByteArray &data)
+{
+    QXmlStreamReader xml(data);
+    if (!xml.readNextStartElement() || xml.name() != QLatin1String("cockatrice_carddatabase")) {
+        return QCoreApplication::translate("OracleDatabase", "The download is not a Cockatrice card database.");
+    }
+    const auto version = xml.attributes().value("version");
+    if (version != QLatin1String("3") && version != QLatin1String("4")) {
+        return QCoreApplication::translate("OracleDatabase", "The downloaded card database version is not supported.");
+    }
+    while (!xml.atEnd()) {
+        xml.readNext();
+    }
+    if (xml.hasError()) {
+        return QCoreApplication::translate("OracleDatabase",
+                                           "The downloaded card database is incomplete or invalid: %1")
+            .arg(xml.errorString());
+    }
+    return {};
+}
+
+bool saveXml(const QString &fileName, const QByteArray &data, QString *error)
+{
+    const QString validationError = validateXml(data);
+    if (!validationError.isEmpty()) {
+        if (error) {
+            *error = validationError;
+        }
+        return false;
+    }
+    QSaveFile file(fileName);
+    if (!file.open(QIODevice::WriteOnly) || file.write(data) != data.size() || !file.commit()) {
+        if (error) {
+            *error = file.errorString();
+        }
+        return false;
+    }
+    if (error) {
+        error->clear();
+    }
+    return true;
+}
+} // namespace OracleDatabase