fix: accept boundary ports in forwarding mappings

[dr-hoseyn]

Summary

• accept the valid boundary ports 1 and 65535 in local=remote forwarding mappings
• centralize numeric-port-to-listen-address conversion for all seven server transports
• preserve explicit IPv4/IPv6 listen addresses

Why

Validation accepts the full 1..65535 port range, but each transport used port > 1 && port < 65535 while parsing local=remote. As a result, mappings such as 1=127.0.0.1:80 and 65535=127.0.0.1:80 were treated as malformed listen addresses at runtime.

Affected transports

TCP, TCPMUX, KCP, QUIC, UDP, WS, and WSMUX.

Verification

• focused helper tests pass natively
• GOOS=linux GOARCH=amd64 go vet ./internal/server/transport
• GOOS=linux GOARCH=amd64 go test -c ./internal/server/transport