Implement NPC extraction follow-up fixes

internal/modules/dnd/npcs/diagnostics/diagnostics_test.go

@@ -0,0 +1,28 @@
+package diagnostics
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"unicode/utf8"
+)
+
+func TestAggregateEnforcesByteBudgetAndReportsOmissions(t *testing.T) {
+	issues := make([]string, MaxIssues)
+	for index := range issues {
+		issues[index] = fmt.Sprintf("issue-%d-%s", index, strings.Repeat("火", MaxDisplayedRunes))
+	}
+
+	message := Aggregate("invalid NPC data", issues)
+	if !utf8.ValidString(message) || len([]byte(message)) > MaxMessageBytes {
+		t.Fatalf("Aggregate() returned invalid or oversized message: bytes=%d message=%q", len([]byte(message)), message)
+	}
+	displayed := strings.Count(message, "issue-")
+	if displayed == 0 || displayed >= len(issues) {
+		t.Fatalf("Aggregate() displayed %d issues, want byte-budget omission", displayed)
+	}
+	wantOmitted := fmt.Sprintf("%d additional issue(s) omitted", len(issues)-displayed)
+	if !strings.Contains(message, wantOmitted) {
+		t.Fatalf("Aggregate() = %q, want %q", message, wantOmitted)
+	}
+}