LSP says about Position::character
> If the character value is greater than the line length it defaults back to the line length.
but from_proto::offset() doesn't implement this.
A client might for example request code actions for a whole line by sending
Position::character=99999. I don't think there is ever a reason (besides laziness) why the
client can't specify the line length instead but I guess we should not crash but follow protocol.
Technically it should be a warning, not an error but warning is not shown by default so keep
it at error I guess.
Fixes#18240