[fix] prerendered endpoint callable from non-prerendered server load

[dummdidumm]

Fixes #8341

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[Rich-Harris]

not even going to try and understand why this works/how it was broken before, but i'm glad the fix is so simple

[dummdidumm]

Good point; this needs a little explanation:
state.initiator wasn't set previously when responding to a __data.json call. If that __data.json call then leads to another internal call (as happened in the bug report), respond runs again (through the internal fetch). It then searches for a manifest entry of that server endpoint, doesn't find one (because it was prerendered), and throws an error if state.initiator isn't set. state.initiator being set doesn't throw the error and instead proceeds to do a native fetch, which is then able to load the prerendered server endpoint.