refactor: Extract document reading to separate class (#127)

This decouples LLMClientInterface from Qt Creator text editor
implementation and allows to write tests

context/IDocumentReader.hpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2025 Povilas Kanapickas
+ *
+ * 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/>.
+ */
+
+#pragma once
+
+#include <QTextDocument>
+
+namespace QodeAssist::Context {
+
+struct DocumentInfo
+{
+    QTextDocument *document = nullptr; // not owned
+    QString mimeType;
+    QString filePath;
+};
+
+class IDocumentReader
+{
+public:
+    virtual ~IDocumentReader() = default;
+
+    virtual DocumentInfo readDocument(const QString &path) const = 0;
+};
+
+} // namespace QodeAssist::Context