Move sidecar port assignment logic to locked stage of add flow

[skpratt]

Description

This relocates automatic sidecar port assignment logic to a locked method immediately before registration, thereby preventing collisions when handling simultaneous requests.

Testing & Reproduction steps

Manual verification:

• Submit many simultaneous requests for registration with sidecar proxy and no port specified: for name in $(shuf -i 1-10000); do curl -d '{"name": "'$name'", "connect": {"sidecar_service": {}}}' -X PUT localhost:8500/v1/agent/service/register &; echo registered $name; done
• See how many duplicate ports have been registered: curl localhost:8500/v1/agent/services | jq -r ' . | to_entries | map(select(.key | match("sidecar";"i"))) | map(.value) | .[] | .Port' | sort | uniq -d
• Before this change: there should be several duplicate ports
• After this change: no duplicate ports

Links

GH Issue

PR Checklist

• [ ] updated test coverage
• [N/A] external facing docs updated
• [N/A] not a security concern