Container `del child_widget` doesn't check if widget has already been removed.

[Antyos]

Describe the bug
Calling del widget on a widget that has already been removed will raise a ValueError.

To Reproduce

from magicgui import widgets


class DeleteTest(widgets.Container):
    def __init__(self, x: int):
        self.x = x
        self.label = widgets.Label(value=f"x = {x}")
        self.delete_button = widgets.PushButton(text="Delete me")
        self.delete_button.clicked.connect(self.delete_label)
        super().__init__(widgets=[self.label, self.delete_button])

    def delete_label(self):
        self.remove(self.label)
        # Now, I don't want the label reference around anymore
        del self.label  # Internally calls self.remove(), but the item is already gone, so it errors


DeleteTest(x=5).show(run=True)

Or even:

d = DeleteTest(x=5)
# x was never a child widget, but this should still be valid
del d.x  # raises ValueError

I think the issue is related to these lines here:

magicgui/src/magicgui/widgets/bases/_container_widget.py

Lines 339 to 345 in 2d985e7

	def remove(self, value: Widget | str) -> None:
	"""Remove a widget instance (may also be string name of widget)."""
	super().remove(value) # type: ignore
	def __delattr__(self, name: str) -> None:
	"""Delete a widget by name."""
	self.remove(name)

Expected behavior
Normal behavior for deling a variable in a mutable sequence after it has been removed does not raise an error.

x = 5
mutable_seq = [1, 2, x]
mutable_seq.remove(x)
# mutable_seq.remove(x)  # Would raise an error if you tried to remove it again.
del x  # But this is fine

print(f"mutable_seq: {mutable_seq}")

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: Windows, Python 3.13.9
• backend: PyQt6==6.9.1
• magicgui version 0.10.3.dev3+g2d985e7a4