fix: join group routes without leading slash

[Rayan-and-beyond]

Fixes #3099

Group routes now insert the missing separator when a group prefix and route path meet without /. Existing routes that already include the separator keep their current behavior.

Validation:

• regression fails on pristine master with /v1posts and 404
• focused group/route tests pass after the fix
• git diff --check

[aldas]

group (prefix) does not necessarly need to mean segment in url that ends with slash.

[Rayan-and-beyond]

That makes sense — the prefix can be an intentional partial path rather than a segment boundary, so inserting a slash would change valid routing semantics. I’ll close this rather than force that behavior. Thanks for clarifying.