Improve unsupported PDF character errors

backend/tests/pdfCharacterError.test.ts

@@ -0,0 +1,27 @@
+import assert from "node:assert/strict"
+import test from "node:test"
+import { getUnsupportedPdfCharacterMessage } from "../src/utils/pdfCharacterError"
+
+test("reports unsupported characters with the document position and character offset", () => {
+    // pdf-lib displays supplementary Unicode characters as a private-use glyph,
+    // while the hexadecimal value still contains the original code point.
+    const error = new Error('WinAnsi cannot encode "" (0x1f600)')
+    const documentData = {
+        rows: [
+            { pos: "1", text: "Montage" },
+            { pos: "2.3", descriptionText: "Gerät 😀 montieren" },
+        ],
+    }
+
+    assert.equal(
+        getUnsupportedPdfCharacterMessage(error, documentData),
+        "Das Zeichen „😀“ (Unicode U+1F600) wird in PDF-Dokumenten nicht unterstützt. Fundstelle: Position 2.3, Beschreibung, Zeichen 7. Bitte ersetze oder entferne das Zeichen."
+    )
+})
+
+test("returns null for unrelated PDF errors", () => {
+    assert.equal(
+        getUnsupportedPdfCharacterMessage(new Error("Background PDF is invalid"), {}),
+        null
+    )
+})