test: rewrite group method-handling tests after review

Addresses review of the original regression file (PR #3003): its comments
described an automatic group catch-all "triggered by middleware" that does
not exist on master (v5), so the tests passed for the wrong reason and the
no-op middleware was inert.

Rewrite to assert v5's actual, verified behavior:
- method mismatch on a group route -> 405 with full Allow header
- OPTIONS on a registered group route -> 204 with Allow (preflight-relevant)
- concrete routes resolve; group prefix does not affect root routes

The 405 and OPTIONS tests are real gates: a group-level catch-all (manual or
the auto-registration proposed in #2996) masks both as 404, which these tests
would then catch. Drops the false premise, the inert middleware, and the
in-comment PR reference.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vishr