fix(onboarding): fail fast on unavailable model endpoints (#160813)

Preserve transport error codes through embedded recovery and let one-shot setup verification reject definite connection failures without spending the normal session retry budget. Keep ordinary agent retries unchanged and report actionable server, URL, and DNS guidance through the shared UI/CLI owner.

Remote proof: 167 focused tests; both regressions fail on baseline. Real Gateway/Chromium medians (five fresh runs): closed port 79.766s to 0.720s, DNS NXDOMAIN 69.449s to 0.849s, HTTP 401 control 1.003s to 0.860s. CLI closed-port verification 66.655s to 1.313s. Successful activation, chat, and reload pass. Changed checks pass after removing the obsolete status-mapper export; the exact dead-export gate and setup tests were rerun after that final cleanup.

Co-authored-by: Peter Steinberger <steipete@gmail.com>

docs/start/onboarding-overview.md

@@ -139,6 +139,11 @@ setup verifies a real model reply before saving the provider and activating its
 model. A failed or cancelled check preserves the previous configuration. The
 classic wizard also retains its custom-provider setup.
 
+If the endpoint refuses the connection or its hostname cannot be found, setup
+reports the failed connection immediately instead of waiting through normal
+chat retries. Start the server or correct the URL and network settings on the
+Gateway host, then retry. Ordinary agent sessions keep their connection retries.
+
 ## Related
 
 - [Getting started](/start/getting-started)