Relay User Report Envelope API is broken

[JoshFerge]

Customers are reporting the user report API is no longer working via javascript SDK.

getsentry/sentry-javascript#10229
getsentry/sentry-javascript#10280
https://www.notion.so/sentry/sentry-captureUserFeedback-Not-Working-d5aa7f6e2fd5416790820a0e2cf0cb47

I have traced this issue to Relay, where git-bisect reveals that the PR #2862 breaks the ingest of user report payloads (note these are the old user reports, captured via the javascript API call Sentry.captureUserFeedback).

The envelope I have used to test this is:

envelope 1 (event creation):

{"event_id":"4cec9f3e1f214073b816e0f4de5f59b1","sent_at":"2024-01-22T17:16:32.564Z","sdk":{"name":"sentry.javascript.browser","version":"7.80.0"},"trace":{"environment":"development","public_key":"ac390a22a11167bbe4318c072a75fdd7","trace_id":"48a06ef0fc6448f69726dc5ac743f1e9"}}
{"type":"event"}
{"message":"User Feedback","level":"info","event_id":"4cec9f3e1f214073b816e0f4de5f59b1","platform":"javascript","request":{"url":"http://localhost:5173/","headers":{"User-Agent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36"}},"timestamp":1705943792.563,"environment":"development","sdk":{"integrations":["InboundFilters","FunctionToString","TryCatch","Breadcrumbs","GlobalHandlers","LinkedErrors","Dedupe","HttpContext"],"name":"sentry.javascript.browser","version":"7.80.0","packages":[{"name":"npm:@sentry/browser","version":"7.80.0"}]},"contexts":{"trace":{"trace_id":"48a06ef0fc6448f69726dc5ac743f1e9","span_id":"932fe0ad83d688bf"}}}

envelope 2 (user report):

{"event_id":"4cec9f3e1f214073b816e0f4de5f59b1","sent_at":"2024-01-22T17:16:32.564Z","sdk":{"name":"sentry.javascript.browser","version":"7.80.0"}}
{"type":"user_report"}
{"name":"Josh","email":"","comments":"I'm having fun","event_id":"4cec9f3e1f214073b816e0f4de5f59b1"}

Note that on that above PR, we get:

TRACE relay_server::actors::processor::event: processing json event
TRACE relay_server::actors::processor::event: no event in envelope
DEBUG relay_server::utils::managed_envelope: dropped envelope: invalid data (no_event_payload)

and on the previous PR: #2936

we get

TRACE relay_server::actors::processor::event: processing json event
TRACE relay_server::actors::processor: sending envelope to kafka
TRACE relay_server::actors::store: Sending kafka message of type user_report