fix for #143 full_value assignment fails when unit present

[rozyczko]

Enhanced Parameter.full_value property:

1. Check if callback returns a scipp Variable or raw number
2. If raw number, convert to scipp scalar with original unit
3. If dimensionless scipp Variable but original has unit, apply original unit
4. Handle UnitError with fallback to preserve original unit and update value only

Also, added a test

[github-actions]

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[henrikjacobsenfys]

How do we know the unit of the callback when it's a raw number? Or more precisely, if it's a raw number, is there ever a situation where the unit of the callback is different from the one in self._scalar? I.e. self._scalar is in Å and the callback gives a numpy.float64 with implied unit nm? (Just trying to think of any way this might break, and this is the only one I could think of - the fix looks good to me)

[rozyczko]

closed due to a better fix in #145