Raise a `ValidationError` for positional-only plans

So we only raise a TypeError when we try and run it? We don't raise a ValidationError while making the model? Can we make a ticket and change this behaviour

_Originally posted by @DiamondJoseph in https://github.com/DiamondLightSource/blueapi/pull/1184#discussion_r2307555319_

We have documented the behaviour when trying to use positional-only arguments in plans https://github.com/DiamondLightSource/blueapi/issues/780. However the error is only thrown when trying to run a plan. It should be thrown when loading the plan on startup.