fix: strict ArgumentCompleter completes on flawed first argument

See the test case for an example. I believe the Argument Matcher should only match any consequent arguments if early arguments are exactly matched, meaning one of the candidates must match excatly the argument.

Sadly, I have no idea how to patch the ArgumentCompleter correctly to do so in all possible cases given the delimiter can be anything, and the candidates may have any kind of delimiter attached at the end.
The tests show the failure, they are not sufficient to validate a correct solution.