fix(entrypoint): reject missing inbound mTLS profile references

Add lookupInboundMTLSProfile so global and route-scoped refs must exist
in the loaded profile map. Propagate resolver errors through TLS
GetConfigForClient; in HTTP dispatch, return 421 only for SNI and
misdirected secure-route cases and log 500 for other resolution
failures.

Support adding routes with an existing listener for tests, reserve the
port via net.Listen without a race, and use t.Cleanup for server
teardown. Move relay_proxy_protocol_header documentation to per-route
TCP config in config.example.yml.

config.example.yml

@@ -160,6 +160,11 @@ providers:
   #     secret: aaaa-bbbb-cccc-dddd
   #     no_tls_verify: true
 
+# To relay the downstream client address to a TCP upstream, set
+# `relay_proxy_protocol_header: true` on that specific TCP route in route
+# configuration (for example, see providers.example.yml). UDP relay is not
+# supported yet.
+
 # Match domains
 # See https://docs.godoxy.dev/Certificates-and-domain-matching
 #