Support `Signal[T] | None` syntax in `DeviceFiller`

[coretl]

We need to support a syntax like this for parameters that are optional, but if they exist should have a given type:

class MyDevice(Device):
    maybe_attr: SignalR[int] | None

The behaviour should be:

• at init then device.maybe_attr is a disconnected SignalR[int]
• at connect then device.maybe_attr should become either become disconnected or become None depending on if the signal exists in the PVI structure

Suggested strategy:

• In DeviceFiller._scan_for_annotations then look for | None and add it to self._optional_devices: set[str]. E.g. go from
  

  ophyd-async/src/ophyd_async/core/_device_filler.py

  Lines 122 to 123 in d381c3b

  	name = UniqueName(attr_name)
  	origin = get_origin_class(annotation)

  
  to

            name = UniqueName(attr_name)
            origin = get_origin_class(annotation)
            args = get_args(annotation)
            if origin is type.UnionType and None in args and len(args) == 2:                
                # We got `T | None`, so register it as optional, and continue with `T`
                self._optional_device.add(name)
                annotation, = [x for x in args if x is not None]
                origin = get_origin_class(annotation)

• In DeviceFiller.check_filled then check those that are not filled but optional to None. E.g. go from
  

  ophyd-async/src/ophyd_async/core/_device_filler.py

  Lines 244 to 248 in d381c3b

  	unfilled = sorted(set(self._unfilled_connectors).union(self._unfilled_backends))
  	if unfilled:
  	raise RuntimeError(
  	f"{self._device.name}: cannot provision {unfilled} from {source}"
  	)

  
  to

        unfilled = set(self._unfilled_connectors).union(self._unfilled_backends)
        unfilled_optional = unfilled.intersection(self._optional_device)
        for name in unfilled_optional:
            setattr(self._device, name, None)
        unfilled_mandatory = sorted(unfilled.difference(unfilled_optional))
        if unfilled_mandatory:            
            raise RuntimeError(
                f"{self._device.name}: cannot provision {unfilled_mandatory} from {source}"
            )

Acceptance Criteria

• Code changed and tests written for PVI devices