fix: identify submitted server version in publish diagnostics (#1621)

Publishing a manifest with a stale version currently produces a
duplicate-version error without identifying the version that was
submitted. This made a request containing `0.4.5` look like a rejected
publication of `0.4.6` in #1615.

Include the submitted `name@version` in the CLI output before sending
the request and in the API duplicate-version error. HTTP 400, the
response structure, and duplicate-rejection behavior remain unchanged.
The service wraps the existing error sentinel so `errors.Is` continues
to work.

Adds a CLI regression test for a failed publication and updates the API
duplicate-error assertion.

Validation:
- Publish command tests passed (`go test ./cmd/publisher/commands -run
TestPublishCommand -count=1`).
- `git diff --check` passed.
- API endpoint tests were attempted but could not run because PostgreSQL
was unavailable on localhost:5432; Docker was also unavailable locally.

Related to #1615.

internal/service/registry_service.go

@@ -190,7 +190,7 @@ func (s *registryServiceImpl) createServerInTransaction(ctx context.Context, tx
 			return e
 		}
 		if versionExists {
-			return database.ErrInvalidVersion
+			return fmt.Errorf("%w: %s@%s already exists", database.ErrInvalidVersion, serverJSON.Name, serverJSON.Version)
 		}
 		currentLatest, e = s.db.GetCurrentLatestVersion(ctx, tx, serverJSON.Name)
 		if e != nil && !errors.Is(e, database.ErrNotFound) {