fix: use gr.State for task_type to prevent stale repaint task leaking into Custom mode

[Copilot]

After using Repaint mode, switching back to Custom mode and generating fails with "Task 'repaint' requires source audio, but none was provided" — task_type still holds "repaint" even though the mode radio shows Custom.

Root cause: task_type was a gr.Textbox(visible=False). In Gradio 6.2.0, mode-switch event outputs targeting hidden textboxes are not reliably applied, leaving stale values from the previous mode visible to the generate button's input read.

Changes

• generation_tab_primary_controls.py — Replace gr.Textbox(visible=False, value="text2music") with gr.State(value="text2music"), which is reliably managed server-side independent of render state.

• mode_ui.py — compute_mode_ui_updates now returns the raw task_type string at index 5 instead of gr.update(value=task_type, elem_classes=[...]). gr.State requires a raw value, not a gr.update() wrapper.

  # Before
  gr.update(value=task_type, elem_classes=["has-info-container"]),  # 5: task_type
  # After
  task_type,  # 5: task_type (gr.State — raw value)

• model_config.py — Change the no-op task_type update in get_model_type_ui_settings from gr.update() to gr.skip(), the correct Gradio 6 idiom for leaving a gr.State unchanged.

• mode_ui_test.py — Add ModeUiTaskTypeTests with regression tests confirming Repaint→Custom resets task_type to "text2music" and that all mode switches return a plain str (not a gr.update() dict).