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/api/handlers/v0/publish_test.go

@@ -168,7 +168,7 @@ func TestPublishEndpoint(t *testing.T) {
 			expectedError:  "You do not have permission to publish this server",
 		},
 		{
-			name: "registry service error",
+			name: "duplicate version includes submitted server identity",
 			requestBody: apiv0.ServerJSON{
 				Schema:      model.CurrentSchemaURL,
 				Name:        "example/test-server",
@@ -202,7 +202,7 @@ func TestPublishEndpoint(t *testing.T) {
 				_, _ = registry.CreateServer(context.Background(), &existingServer)
 			},
 			expectedStatus: http.StatusBadRequest,
-			expectedError:  "invalid version: cannot publish duplicate version",
+			expectedError:  "invalid version: cannot publish duplicate version: example/test-server@1.0.0 already exists",
 		},
 		{
 			name: "package validation success - MCPB package",