MINOR: Refine route-acl rules to prevent unintended prefix matches

[fabianonunes]

Since route-acl annotated rules take precedence over others, this PR updates its behavior to ensure they do not unintentionally overwrite other rules that share the same word prefix.

For example, a rule matching the path prefix /api should not inadvertently handle requests to /apiary.

To address this, the rule { path -m beg /api } has been replaced with a alternative that validates the URL's termination, ensuring it matches only /api$ or /api/.*:

# before this PR:
use_backend app_api_http if { var(txn.host) -m str api.demo } { path -m beg /api } { ... }

# after:
use_backend app_api_http if { var(txn.host) -m str api.demo } { path -m reg ^/api($|/) } { ... }

For better maintainability, this PR also refactors the AddCustomRoute function to eliminate redundancy introduced in commit c28d620. The updated code removes repetition without add extra spaces.