Extracted widget code into own class - #27

Please address the comments from this code review:

## Overall Comments
- In `GtpyPythonNodeWidget::create`, returning an empty `unique_ptr` when the node is not a `GtpyPythonNode` may violate `registerWidgetFactory` expectations; consider asserting the type or providing a minimal fallback widget instead of returning null.
- In `gtpn_pythonnodewidget.cpp` headers like `gtpy_contextmanager.h`, `gtpy_gilscope.h`, and `gtpy_tempdir.h` are included but not used in this file; they can likely be removed to reduce compile-time dependencies.

## Individual Comments

### Comment 1
<location path="src/gui/gtpn_pythonnodewidget.cpp" line_range="74-83" />
<code_context>
+    }
+}
+
+void
+GtpyPythonNodeWidget::updateTimePassed(int progress)
+{
+    if (progress != 100)
+    {
+        m_button->setIcon(gt::gui::icon::processRunningIcon(progress));
+    }
+    else
+    {
+        m_button->setIcon(gt::gui::icon::python());
+    }
+}
+
+void
+GtpyPythonNodeWidget::showCustomContextMenu(const QPoint& pos)
+{
+    if (!m_textEdit)
+    {
+        return;
+    }
+
+    QMenu* menu = m_textEdit->createStandardContextMenu();
+    QAction* customAction = new QAction(QStringLiteral("Assistant"), menu);
+
+    connect(customAction, &QAction::triggered,
+            this, &GtpyPythonNodeWidget::insertAssistantPrompt);
+
+    menu->addSeparator();
+    menu->addAction(customAction);
+    menu->exec(m_textEdit->mapToGlobal(pos));
+    delete menu;
+}
+
+void
+GtpyPythonNodeWidget::insertAssistantPrompt()
+{
+    if (!m_textEdit)
</code_context>
<issue_to_address>
**issue (bug_risk):** The Assistant menu installation is guarded by a different capability than the prompt invocation, which may lead to a no-op action.

`openScriptEditor()` installs the context menu based on `getSharedFunction("Assistant", "prompt")`, but `insertAssistantPrompt()` calls `getSharedFunction("Assistant", "open_prompt_python")`. If only one of these is available, the "Assistant" menu will appear but do nothing when clicked. Consider basing the menu installation on the availability of `open_prompt_python`, or using a single consistent capability key for both the check and the call.
</issue_to_address>