ref(upstream): Forward envelopes to the advertised upstream

[Dav1dde]

Implements forwarding of envelopes to the advertised upstream of an upstream Relay.

This adds some integration test machinery to allow observing http requests (mini_proxy).

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Host header not updated for overridden upstream

High Severity

When a request provides an upstream override, the URL is correctly resolved from the overridden UpstreamDescriptor, but the Host header fallback still always uses self.config.upstream().host(). This means requests routed to a different upstream will carry the wrong Host header, which can cause routing failures at the target server in production where hosts differ. The fallback in host_header needs to also consider request.upstream().

Additional Locations (1)

• relay-server/src/services/upstream.rs#L897-L901

 

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[Dav1dde]

Same as on the Sentry one: I actually kept this for now on purpose, didn't see a reason to override it.

[loewenheim]

This is without having read the proxy testing logic in detail.