fix: keep shared server config when deleting one of multiple contexts

[Syedowais312]

Summary

This fixes context deletion when multiple contexts reference the same server entry.

If the config contains a single server and multiple contexts pointing to it,
deleting one context could also remove shared metadata that was still needed by the
remaining context.

Changes

• resolve the target context before deleting it
• delete only the selected context
• remove shared user, server, auth, and instance entries only when no
  remaining context references them
• add a regression test for multiple contexts sharing one server

Reproducer

./microcks start
./microcks login http://localhost:8585 --name dev
./microcks context dev --delete

Testing

  go test ./cmd ./pkg/config

closes: #421