Feature/endpoint substring fix

[gedge]

the below text is shamelessly stolen from #150 but my PR includes tests 😁

What

Proxies were being used when there was a substring match (e.g. /files proxy was being used for /filesize requests) so the string builds have been updated to ensure the following logic:

• either: end of string exactly
• or: the remaining segment must begin with a /, and could be any length

This would mean in this example /files, /files/ and /files/id would all resolve, but /filesize would not

How to review

Ensure existing page types being routed through these 2 proxy string builders continue to work

• test both publishing and web journeys!

Who can review

anyone