Add regression tests for invoice validation and CSV parsing

tests/ImportAssist.Core.Tests/ImportAssist.Core.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\ImportAssist.Core\ImportAssist.Core.csproj" />
+  </ItemGroup>
+
+</Project>

tests/ImportAssist.Core.Tests/InvoiceCsvParserTests.cs

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+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);
+    }
+}

tests/ImportAssist.Core.Tests/InvoiceValidatorTests.cs

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+using System.Globalization;
+using ImportAssist.Core;
+using Xunit;
+
+namespace ImportAssist.Core.Tests;
+
+public sealed class InvoiceValidatorTests
+{
+    [Theory]
+    [InlineData("de-DE")]
+    [InlineData("en-US")]
+    public void Validate_AcceptsDecimalPointRegardlessOfCurrentCulture(
+        string cultureName)
+    {
+        var originalCulture = CultureInfo.CurrentCulture;
+
+        try
+        {
+            CultureInfo.CurrentCulture = new CultureInfo(cultureName);
+            var row = new InvoiceRow(
+                "INV-001", "C-100", "125.50", "EUR");
+
+            var errors = InvoiceValidator.Validate(row);
+
+            Assert.Empty(errors);
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = originalCulture;
+        }
+    }
+
+    [Theory]
+    [InlineData("abc", "Betrag ist keine gültige Zahl.")]
+    [InlineData("125,50", "Betrag ist keine gültige Zahl.")]
+    [InlineData("1,250.00", "Betrag ist keine gültige Zahl.")]
+    [InlineData("0", "Betrag muss größer als null sein.")]
+    [InlineData("-10.00", "Betrag muss größer als null sein.")]
+    public void Validate_RejectsInvalidAmount(
+        string amount,
+        string expectedError)
+    {
+        var row = new InvoiceRow(
+            "INV-001", "C-100", amount, "EUR");
+
+        var errors = InvoiceValidator.Validate(row);
+
+        Assert.Equal(expectedError, Assert.Single(errors));
+    }
+
+    [Fact]
+    public void Validate_CollectsAllErrorsForOneInvoice()
+    {
+        var row = new InvoiceRow(" ", "\t", "0", "EUR");
+
+        var errors = InvoiceValidator.Validate(row);
+
+        Assert.Equal(3, errors.Count);
+        Assert.Contains("Rechnungsnummer fehlt.", errors);
+        Assert.Contains("Kundennummer fehlt.", errors);
+        Assert.Contains("Betrag muss größer als null sein.", errors);
+    }
+}