This commit introduces a suite of tests for the drag and drop functionality
in `sub_plot_widget.py`. The tests cover the following scenarios:

1.  Dragging a new data item (e.g., from a variable list) onto a
    SubPlotWidget to create a new trace.
2.  Reordering an existing trace (CustomPlotItem) within the same
    SubPlotWidget by dragging its label.
3.  Moving an existing trace from one SubPlotWidget to another. This
    includes verifying that signal connections for time synchronization
    are updated.
4.  Dragging a trace label onto the main plot area (not the label area)
    of the same SubPlotWidget, which should append it to the end of the
    trace list.
5.  Attempting to drag items with unrecognized mime types, ensuring these
    are correctly ignored by the SubPlotWidget.

Helper functions for creating mock Qt drag/drop events, mime data, and
for common test setup and assertion patterns have been added to
`tests/test_sub_plot_widget.py` to support these tests and improve
maintainability.

This commit refactors the drag and drop tests for SubPlotWidget
in `tests/test_sub_plot_widget.py` to utilize `pytest-qt` more
thoroughly for event simulation.

Key changes include:
- Modified `custom_plot_item.py` to make `CustomPlotItem` instances
  genuinely draggable by implementing `mousePressEvent` and
  `mouseMoveEvent` to initiate a `QDrag` operation with appropriate
  mime data.
- Introduced new mock draggable items (`MockDraggableVarItem`,
  `MockDraggableUnrecognizedItem`) in the test suite to simulate
  drag origins for different scenarios. These mocks also initiate `QDrag`.
- All drag-and-drop test cases now use `qtbot.mousePress` and
  `qtbot.mouseMove` on these draggable items/widgets to start the
  drag operation.
- `QDrag.exec_()` is mocked for these tests. The side_effect of this
  mock dispatches `QDragEnterEvent`, `QDragMoveEvent` (optional),
  and `QDropEvent` to the `SubPlotWidget` using
  `QApplication.sendEvent()`. The `source()` of these dispatched
  events is patched to ensure the target widget correctly identifies
  the drag originator.
- Obsolete helper functions for manual event creation have been removed.

Note: During execution in the automated environment, I encountered a "Fatal Python error: Aborted" during the `qapp` fixture initialization (part of `pytest-qt`). This prevented full verification of these refactored tests in that specific environment. The code has been structured for compatibility with a functional `pytest-qt` setup.

This commit refactors the drag-and-drop (DND) tests for SubPlotWidget
in `tests/test_sub_plot_widget.py` to utilize `pytest-qt` more
thoroughly for event simulation.

Key changes include:

1.  **Draggable `CustomPlotItem`:**
    *   I modified `custom_plot_item.py` to make `CustomPlotItem` instances
        genuinely draggable. They now implement `mousePressEvent` and
        `mouseMoveEvent` to initiate a `QDrag` operation with the
        "application/x-customplotitem" mime type.

2.  **Test Implementation with `pytest-qt`:**
    *   All DND tests now use `qtbot.mousePress` and `qtbot.mouseMove`
        on the source items (either a `CustomPlotItem` or a new mock
        draggable widget) to initiate drag operations.
    *   I introduced new mock widgets (`MockDraggableVarItem`,
        `MockDraggableUnrecognizedItem`) to simulate
        specific drag sources.
    *   `QDrag.exec_()` is consistently mocked. Its side_effect function
        simulates the DND event sequence on the `SubPlotWidget` by
        dispatching events (QDragEnterEvent, QDragMoveEvent, QDropEvent)
        via `QApplication.sendEvent()`.
    *   I patched the `source()` method of these dispatched events to ensure
        the `SubPlotWidget` correctly identifies the drag originator.

3.  **Helper Function Cleanup:**
    *   I removed obsolete helper functions previously used for manual event
        creation. Mime data creation helpers have been retained.

This approach makes the DND tests more realistic by having `qtbot`
drive user input and by making the application's `CustomPlotItem`
an active participant in the drag operation.

Note: I encountered a fatal Qt/pytest-qt setup error that hindered test execution in the remote environment. I'm providing these changes for your local testing and review.