cli/compose/loader: don't drop tcp port when udp uses same published port

[UditDewan]

- What I did

Fixed docker stack config / docker stack deploy silently dropping a port entry when merging multiple compose files where a service publishes the same port on both tcp and udp (e.g. 443:443 and 443:443/udp). The entry was lost even when the override file did not define any ports.

Fixes #6223

- How I did it

During merge, ports were indexed by their published port only, so the tcp and udp entries for the same published port collided and the later one overwrote the earlier one. The merge key now includes the protocol (an omitted protocol is treated as tcp, matching the long-syntax default), and the sort of the merged result uses the protocol as a tie-breaker so the output order is deterministic.

- How to verify it

# compose.yaml
services:
  web:
    image: caddy
    ports:
      - "80:80"
      - "443:443"
      - "443:443/udp"

# compose.override.yaml
services:
  web:
    environment:
      FOO: bar

docker stack config -c compose.yaml -c compose.override.yaml now returns all three ports; previously the 443:443/tcp entry was missing. Regression tests added in cli/compose/loader/merge_test.go (go test ./cli/compose/loader/ -run TestLoadMultipleServicePorts).

- Human readable description for the release notes

Fix `docker stack config` dropping a port when merging compose files that publish the same port on both tcp and udp