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/Session.hpp

@@ -0,0 +1,78 @@
+// 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 <QList>
+#include <QObject>
+#include <QPointer>
+#include <QString>
+
+#include <functional>
+#include <optional>
+
+#include "session/ChatBackend.hpp"
+#include "session/ConversationHistory.hpp"
+#include "session/HistoryProjection.hpp"
+#include "session/SessionEvent.hpp"
+#include "session/TurnContext.hpp"
+#include "session/TurnRequest.hpp"
+
+namespace QodeAssist::Session {
+
+class Session : public QObject
+{
+    Q_OBJECT
+
+public:
+    explicit Session(QObject *parent = nullptr);
+
+    void setBackend(ChatBackend *backend);
+
+    const ConversationHistory &history() const;
+    const QList<MessageRow> &rows() const;
+
+    void setHistory(const ConversationHistory &history);
+    void clear();
+    void truncateRows(int rowIndex);
+
+    void sendTurn(
+        const QList<ContentBlock> &userBlocks,
+        const std::optional<TurnContext> &context,
+        const TurnOptions &options);
+    void cancel();
+
+    bool updateFileEditStatus(
+        const QString &editId, const QString &status, const QString &statusMessage = {});
+
+signals:
+    void rowsAppended(const QList<QodeAssist::Session::MessageRow> &rows);
+    void rowUpdated(int index, const QodeAssist::Session::MessageRow &row);
+    void rowsRemoved(int first, int count);
+    void rowsReset(const QList<QodeAssist::Session::MessageRow> &rows);
+    void turnStarted(const QString &turnId);
+    void turnFinished(const QString &turnId);
+    void turnFailed(const QString &error);
+    void usageReceived(const QodeAssist::Session::Usage &usage);
+
+private:
+    void handleEvent(const SessionEvent &event);
+    void appendMessage(const Message &message);
+    Message *activeAssistantMessage();
+    void mutateAssistant(const std::function<void(Message &)> &mutate);
+    void mutateAssistantTail(const std::function<void(Message &)> &mutate);
+    void ensureAssistantMessage();
+    void updateLastAssistantRow();
+    void syncAssistantRows();
+    void endTurn();
+
+    ConversationHistory m_history;
+    QList<MessageRow> m_rows;
+    QPointer<ChatBackend> m_backend;
+    QString m_activeTurnId;
+    QString m_textSegment;
+    int m_assistantRowStart = -1;
+};
+
+} // namespace QodeAssist::Session