Match Devise mappings by path segment

[OskarEichler]

Summary

• match mappings on complete URL path segments instead of arbitrary substrings
• prevent an earlier users mapping from capturing an OmniAuth failure under /superusers/auth/...
• preserve nested prefixes, multi-segment full paths, and the existing empty-path behavior

Devise::Mapping.find_by_path! is used by the default OmniAuth failure handler. With users registered before superusers, the current substring lookup resolves /superusers/auth/github/callback to users and can dispatch failure handling through the wrong mapping/controller.

Verification

• baseline reproduction resolves /superusers/auth/github/callback to user
• fixed main and release-based models resolve it to superuser
• prefixed /localized/users/... still resolves to user
• /superusers_extra/... no longer partially matches
• full Active Record suite: 878 runs, 2,470 assertions, 0 failures
• release-based suite: 878 runs, 2,469 assertions, 0 failures
• Ruby syntax check passes

No public API or route format changes are intended; only partial-segment matches are rejected. No test files were changed. Prepared with Codex assistance.