[fix] goto self can incorrectly result in 404 #4316

[aolose]

If shadow page is the last matching route and endpoint return 204, client will keep fallthrough to next, so you will get 404.

fix: #4316

test:

cd packages\kit\test\apps\basics
npx  playwright test -g 'The last matching route fallthrough'

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[Rich-Harris]

thank you!