fix(types): name the clashing service and make the error stable

[AlbertEinsteinTG]

Problem

When two services declare the same container_name, the error names neither
the conflicting service nor anything actionable:

services.a: container name "shared" is already in use by service {}"

Two defects in one line:

1. {} instead of the other service's name.
2. A stray unbalanced " at the end.

And a third, related issue: the message is not deterministic. The same
project reports a different service on each run.

Reproduce

services:
  a:
    image: alpine
    container_name: shared
  b:
    image: alpine
    container_name: shared

docker compose config, six consecutive runs, no changes in between:

services.a: container name "shared" is already in use by service {}"
services.a: container name "shared" is already in use by service {}"
services.a: container name "shared" is already in use by service {}"
services.b: container name "shared" is already in use by service {}"
services.b: container name "shared" is already in use by service {}"
services.a: container name "shared" is already in use by service {}"

Docker Compose v5.1.3 (vendoring compose-go v2.10.2). Reproduces on main.

Cause

types/project.go:

names := utils.Set[string]{}
for name, s := range p.Services {
    if s.ContainerName != "" {
        if existing, ok := names[s.ContainerName]; ok {
            return fmt.Errorf(`services.%s: container name %q is already in use by service %s"`, name, s.ContainerName, existing)
        }
        names.Add(s.ContainerName)
    }
}

utils/set.go:

type Set[T comparable] map[T]struct{}

existing is the map value — struct{}{} — not the service that claimed
the name, and %s renders an empty struct as {}. A Set structurally cannot
carry the owning service, so this never worked. The trailing " looks like a
leftover from "%s" that lost its opening quote.

The non-determinism comes from range p.Services: Go randomises map iteration,
so which service is recorded first, and therefore which is reported as the
offender, varies per run.

Change

• Track container_name -> declaring service in a map[string]string so the
  conflicting service can be named.
• Print it with %q, which supplies the quotes and drops the stray one.
• Iterate via ServiceNames() (already in this file, returns sorted names) so
  the reported pair is stable.

After:

services.b: container name "shared" is already in use by service "a"

Tests

The existing assertion stopped one word before the broken segment:

assert.Assert(t, strings.Contains(err.Error(), `container name "mycontainer" is already in use by`))

so everything after is already in use by was unverified — which is how both
defects survived. It now asserts the full message, which the stable ordering
makes possible.

Notes

• No API change. CheckContainerNameUnicity's signature and error semantics are
  unchanged; only the message text and its stability.
• The determinism half mirrors
  fix(interpolation): report all errors in a deterministic order —
  same root cause (randomised map iteration producing a different message per
  run). Happy to split this into two commits if you'd prefer them reviewed
  separately.