Function relay_inject 

pub(crate) async fn relay_inject(
    base_url: &str,
    http: &Client,
    token: &str,
    source_agent: &str,
    target_agent: &str,
    message: &str,
    priority: &str,
    wan: Option<&WanCarried>,
) -> RelayOutcome

POST one injection to the cloud relay.

Deliberately takes base_url and token rather than resolving them internally: that keeps this function a pure HTTP operation with no globals, so the tests can point it at a stub relay without mutating process-wide env (which would race the rest of the test binary).

Contract (verified against the cloud relay)

• source_agent travels in the X-Agent-ID header, not the body — the route 400s without it and derives the sender from it alone.
• X-Client-Wrapped: true is required. Without it the cloud wraps the message in its own [JEKT:...] marker before storing; the receiving srv then wraps it again in Handler::inject_message, so the recipient sees a doubled marker. Sending raw + telling the cloud not to wrap yields exactly one marker, applied by the receiver — identical to how tiers 2 and 3 already behave.