refactor: Extract performance logging to separate class (#124)

This should not be responsibility of LLMClientInterface. Extracting this
class also adds flexibility to silence logging output in tests.

logger/RequestPerformanceLogger.hpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2025 Povilas Kanapickas <povilas@radix.lt>
+ *
+ * This file is part of QodeAssist.
+ *
+ * QodeAssist is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * QodeAssist is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with QodeAssist. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include "IRequestPerformanceLogger.hpp"
+#include <QDateTime>
+#include <QMap>
+
+namespace QodeAssist {
+
+class RequestPerformanceLogger : public IRequestPerformanceLogger
+{
+public:
+    RequestPerformanceLogger() = default;
+    ~RequestPerformanceLogger() override = default;
+
+    void startTimeMeasurement(const QString &requestId) override;
+    void endTimeMeasurement(const QString &requestId) override;
+    void logPerformance(const QString &requestId, qint64 elapsedMs) override;
+    void logPerformance(const QString &requestId, const QString &operation, qint64 elapsedMs);
+
+private:
+    QMap<QString, qint64> m_requestStartTimes;
+};
+
+} // namespace QodeAssist