fix(dock): close legacy tray tooltip on popup destruction

[svan71]

What changed

Close a legacy tray tooltip directly when its PluginPopup emits
aboutToDestroy.

Fixes linuxdeepin/developer-center#13616.

Root cause

Legacy tray tooltip content is owned by a plugin popup while the translucent
tooltip host is owned by dde-shell. The plugin loader correctly hides and
destroys its popup, but ShellSurfaceItemProxy.onSurfaceDestroyed is not emitted
for this teardown. TrayItemSurfacePopup therefore never closes the shared host,
leaving an empty translucent background after the text disappears.

PluginPopup.aboutToDestroy is the authoritative server-side lifecycle signal
and is emitted immediately before the popup object is deleted. Connecting the
tooltip to that signal closes the host at the correct boundary.

Impact

Legacy tray items such as Volume and Clipboard dismiss their tooltip background
correctly. Native actions such as Quick Actions and Collapse Tray are unchanged.

Validation

• Captured the complete plugin lifecycle: pointer leave, QWidget/QWindow hide,
  popup-map removal, and PluginPopupSurface destruction all completed.
• Confirmed the old ShellSurfaceItemProxy.onSurfaceDestroyed handler did not
  run for the plugin popup teardown.
• Built a clean Arch package from dde-shell 2.0.50 containing only this lifecycle
  connection.
• Live-tested Volume and Clipboard with the exact straight-down pointer exit;
  both now dismiss cleanly.
• Rebuilt without diagnostic logging and repeated the live test successfully.

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[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: svan71

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[deepin-ci-robot]

Hi @svan71. Thanks for your PR. 😃

[deepin-ci-robot]

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