Add regression tests for invoice validation and CSV parsing

tests/ImportAssist.Core.Tests/InvoiceCsvParserTests.cs

@@ -0,0 +1,55 @@
+using ImportAssist.Core;
+using Xunit;
+
+namespace ImportAssist.Core.Tests;
+
+public sealed class InvoiceCsvParserTests
+{
+    private const string Header =
+        "InvoiceId;CustomerId;Amount;Currency";
+
+    [Fact]
+    public void Parse_PreservesEmptyFieldsAndRawAmount()
+    {
+        var csv = $"{Header}\nINV-001;;abc;EUR";
+
+        var rows = InvoiceCsvParser.Parse(csv);
+
+        var row = Assert.Single(rows);
+
+        Assert.Equal(
+            new InvoiceRow("INV-001", "", "abc", "EUR"),
+            row);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("InvoiceId;Amount")]
+    public void Parse_RejectsMissingOrIncorrectHeader(string csv)
+    {
+        var exception = Assert.Throws<FormatException>(
+            () => InvoiceCsvParser.Parse(csv));
+
+        Assert.Equal(
+            "Die Datei ist leer oder die Kopfzeile stimmt nicht.",
+            exception.Message);
+    }
+
+    [Theory]
+    [InlineData("INV-002;C-200;89.00")]
+    [InlineData("INV-002;C-200;89.00;EUR;extra")]
+    [InlineData("INV-002;\"C-200\";89.00;EUR")]
+    public void Parse_RejectsMalformedRowWithCorrectLineNumber(
+        string malformedRow)
+    {
+        var csv =
+            $"{Header}\nINV-001;C-100;125.50;EUR\n{malformedRow}";
+
+        var exception = Assert.Throws<FormatException>(
+            () => InvoiceCsvParser.Parse(csv));
+
+        Assert.Equal(
+            "Zeile 3: Erwartet werden vier Spalten ohne Anführungszeichen.",
+            exception.Message);
+    }
+}