fix(cli): skip redundant network create

[7ttp]

TL;DR

fixes supabase start --network-id failing with failed to create docker network
when the network already exists but the create request dies in transit, for example an EPIPE through a docker socket forwarder.

was caused by legacyEnsureNetwork always spawning a docker network create that is a guaranteed conflict for an existing network, and is now fixed by:
probing docker network inspect first and only creating on a miss, the same shape ensureDockerNetwork already uses. The shared bootstrap path also covers db start and db reset....

ref:

• closes: CLI-2151
• closes: Supabase CLI 2.109.1: later network-create gets EPIPE through Unix-socket forwarder on GitHub Actions Ubuntu 24.04 #6127