fix(gateway): keep tool errors from poisoning upstream health

[jmagar]

Summary

• treat successful MCP CallToolResult responses as connection-health successes even when is_error=true
• allow disabled protected routes to retain stale target references without blocking startup
• fall back to legacy initialization when server/discover returns JSON-RPC method-not-found (-32601)
• fail open when a locally built binary cannot access its compile-time built-in snippets path under production ProtectHome
• update the discovery metadata compatibility test for rmcp 3.1 behavior

Verification

• focused Code Mode health regression passes
• focused disabled-route startup regression passes
• lifecycle compatibility and transport fallback regressions pass
• inaccessible snippets-path regression demonstrated red, then passes after the fix
• config suite: 78 passed
• gateway relevant tests pass; two load-sensitive provider tests that timed out in a broad concurrent run passed individually
• cargo fmt --all --check
• git diff --check
• all-features UI-embedded build deployed and live-verified
• live Code Mode search discovers Unifi::unifi; its safe health action returns meta.rc=ok
• live UniFi invalid_param result leaves runtime connected with no last error

Runtime hardening

Production systemd now uses Restart=always with a 3-second delay. A controlled SIGTERM test changed the main PID and returned /ready successfully. Explicit systemctl stop remains authoritative.