Bounded-context error for the built-in proxy crate’s lifecycle and setup
path: building the Tokio runtime, validating proxy configuration, binding the
listener on the configured host and port, running the HTTP server, and
discovering the upstream prefill/decode backends it fronts. Each variant
preserves the kind of failure
plus the actionable detail, following the hand-rolled ConfigError
convention used by Inferlab rather than collapsing every failure into one
opaque string at the library boundary.