[fix] overly restrictive route breakout regex

[Conduitry]

The regex for validating + files was overly restrictive and did not allow route layout breakout to [foo] or (foo) directories. No tests, yet.

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[Conduitry]

check seems to be failing with the same sort of error as in #6218. I don't know why this is. It's passing for me locally.

[Rich-Harris]

failure is probably related to #6222