Create a content provider instance to get message content models from

This means that all content models will now come from the same source to remove duplication across multiple models and `chatbarcaches`.

It also handily breaks the dependency on needing `MessageContentModel` for `NeochatRoom`

src/contentprovider.h

@@ -0,0 +1,67 @@
+// SPDX-FileCopyrightText: 2025 James Graham <james.h.graham@protonmail.com>
+// SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+
+#pragma once
+
+#include <QCache>
+
+#include "models/messagecontentmodel.h"
+#include "models/threadmodel.h"
+#include "neochatroom.h"
+
+/**
+ * @class ContentProvider
+ *
+ * Store and retrieve models for message content.
+ */
+class ContentProvider
+{
+public:
+    /**
+     * Get the global instance of ContentProvider.
+     */
+    static ContentProvider &self();
+
+    /**
+     * @brief Returns the content model for the given event ID.
+     *
+     * A model is created if one doesn't exist. Will return nullptr if evtOrTxnId
+     * is empty.
+     *
+     * @warning If a non-empty ID is given it is assumed to be a valid Quotient::RoomMessageEvent
+     *          event ID. The caller must ensure that the ID is a real event. A model will be
+     *          returned unconditionally.
+     *
+     * @warning Do NOT use for pending events as this function has no way to differentiate.
+     */
+    MessageContentModel *contentModelForEvent(NeoChatRoom *room, const QString &evtOrTxnId, bool isReply = false);
+
+    /**
+     * @brief Returns the content model for the given event.
+     *
+     * A model is created if one doesn't exist. Will return nullptr if event is:
+     *  - nullptr
+     *  - not a Quotient::RoomMessageEvent (e.g a state event)
+     *
+     * @note This method is preferred to the version using just an event ID as it
+     *       can perform some basic checks. If a copy of the event is not available,
+     *       you may have to use the version that takes an event ID.
+     *
+     * @note This version must be used for pending events as it can differentiate.
+     */
+    MessageContentModel *contentModelForEvent(NeoChatRoom *room, const Quotient::RoomEvent *event, bool isReply = false);
+
+    /**
+     * @brief Returns the thread model for the given thread root event ID.
+     *
+     * A model is created if one doesn't exist. Will return nullptr if threadRootId
+     * is empty.
+     */
+    ThreadModel *modelForThread(NeoChatRoom *room, const QString &threadRootId);
+
+private:
+    ContentProvider();
+
+    QCache<QString, MessageContentModel> m_eventContentModels;
+    QCache<QString, ThreadModel> m_threadModels;
+};