Fix: Middleware does not match when use :path (#72056)
For a given path expression, we want it to optionally match a `.json`
suffix. However, the original implementation was flawed:
The path `/:id` was altered to `/:id(.json)?`, which then converted to
the Regex pattern `^(?:\/(.json))?[\/#\?]?$`. This Regex incorrectly
matched only `/` or `/_json` — clearly not the intended result.
The fix involved adjusting the way we append the `.json` suffix. Instead
of `/:id(.json)?`, we now use `/:id{(\\.json)}?`, which converts to the
Regex `^(?:\/([^\/#\?]+?))(\.json)?[\/#\?]?$`. This pattern correctly
matches both cases, with or without the `.json` suffix.
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Fixes https://github.com/vercel/next.js/issues/53840
Fixes https://github.com/vercel/next.js/issues/55558
Closes DX-2088
