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.

cmd/publisher/commands/publish.go

@@ -64,7 +64,7 @@ func PublishCommand(args []string) error {
 	}
 
 	// Publish to registry
-	_, _ = fmt.Fprintf(os.Stdout, "Publishing to %s...\n", registryURL)
+	_, _ = fmt.Fprintf(os.Stdout, "Publishing %s@%s to %s...\n", serverJSON.Name, serverJSON.Version, registryURL)
 	response, statusCode, err := publishToRegistry(registryURL, serverData, token)
 	if err != nil {
 		// If publish failed with 422, call validate endpoint to show detailed errors

cmd/publisher/commands/publish_test.go

@@ -39,6 +39,36 @@ func TestPublishCommand_Success(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestPublishCommand_PrintsSubmittedIdentityOnFailure(t *testing.T) {
+	server := SetupMockRegistryServer(t, func(w http.ResponseWriter, _ *http.Request) {
+		http.Error(w, "duplicate version", http.StatusBadRequest)
+	}, nil)
+	SetupTestToken(t, server.URL, "test-token")
+	CreateTestServerJSON(t, apiv0.ServerJSON{
+		Schema:      model.CurrentSchemaURL,
+		Name:        "io.github.BargLabs/cejel",
+		Description: "A test server",
+		Version:     "0.4.5",
+	})
+
+	output, err := os.CreateTemp(t.TempDir(), "stdout")
+	require.NoError(t, err)
+	originalStdout := os.Stdout
+	os.Stdout = output
+	t.Cleanup(func() {
+		os.Stdout = originalStdout
+		_ = output.Close()
+	})
+
+	err = commands.PublishCommand(nil)
+	require.ErrorContains(t, err, "duplicate version")
+	_, err = output.Seek(0, io.SeekStart)
+	require.NoError(t, err)
+	data, err := io.ReadAll(output)
+	require.NoError(t, err)
+	assert.Equal(t, "Publishing io.github.BargLabs/cejel@0.4.5 to "+server.URL+"...\n", string(data))
+}
+
 func TestPublishCommand_PreservesNonASCIIDescription(t *testing.T) {
 	server := SetupMockRegistryServer(t,
 		func(w http.ResponseWriter, r *http.Request) {