refactor: Decoupling chat architecture (#367)

* refactor: Migrate tests to Qt Creator plugin framework and update llmqore transports

* refactor: Move conversation history into IDE-free session library

* refactor: Extract turn context assembly behind injected ports

* build: Add RunQodeAssistTests target for the plugin test suite

* refactor: Route the chat through a Session and ChatBackend seam

sources/session/ChatBackend.hpp

@@ -0,0 +1,28 @@
+// Copyright (C) 2026 Petr Mironychev
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Additional attribution terms under GPLv3 §7(b) apply — see LICENSE
+
+#pragma once
+
+#include <QObject>
+
+#include "session/SessionEvent.hpp"
+#include "session/TurnRequest.hpp"
+
+namespace QodeAssist::Session {
+
+class ChatBackend : public QObject
+{
+    Q_OBJECT
+
+public:
+    using QObject::QObject;
+
+    virtual void sendTurn(const TurnRequest &request) = 0;
+    virtual void cancel() = 0;
+
+signals:
+    void sessionEvent(const QodeAssist::Session::SessionEvent &event);
+};
+
+} // namespace QodeAssist::Session