Implement ProtocolPort.MarshalJSON to produce string format matching
UnmarshalJSON expectations (e.g. "tcp:443", "udp:10000-20000", "*").
Add comprehensive TestGrantMarshalJSON with 10 test cases:
- IP-based grants with TCP, UDP, ICMP, and wildcard protocols
- Single ports, port ranges, and wildcard ports
- Capability-based grants using app field
- Grants with both ip and app fields
- Grants with via field for route filtering
- Testing omitempty behavior for ip, app, and via fields
- JSON round-trip validation (marshal → unmarshal → compare)
Add omitempty tag to Grant.InternetProtocols to avoid marshaling
null when field is empty.
Updates #2180
90c9555876