segment cache: fix segment cache normalizer (#90111)
This fixes an issue where deploying an app that explicitly named a
catch-all segments as `[...segments]` or `[[...segments]]` would lead to
repeated ISR cache misses on that route, even if it was generated at
build time.
This is because the normalizer that is responsible for extracting the
segment path wasn't escaping the `.segments` directory value, so the `.`
was treated as a wildcard. That meant when the server attempted to look
up the underlying segment data, it wouldn't find it and would have
responded with a 204, which signals to the upstream CDN that it is not a
cacheable response.
Fixes #87090
