moq-relay: refactor auth error handling, add HTTP client tests (#1311)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kixelated

Cargo.lock

@@ -64,4 +64,6 @@ url = { version = "2", features = ["serde"] }
 sd-notify = "0.5"
 
 [dev-dependencies]
+rcgen = "0.13"
 tempfile = "3"
+wiremock = "0.6"