Add structured AI analysis output and invoice ID validation

tests/ImportAssist.Core.Tests/InvoiceAnalysisValidatorTests.cs

@@ -0,0 +1,40 @@
+using ImportAssist.Core;
+using Xunit;
+
+namespace ImportAssist.Core.Tests;
+
+public sealed class InvoiceAnalysisValidatorTests
+{
+    [Theory]
+    [InlineData(null, true)]
+    [InlineData("INV-001", true)]
+    [InlineData("", false)]
+    [InlineData(" ", false)]
+    [InlineData("\t", false)]
+    [InlineData("\r\n", false)]
+    public void Validate_ChecksInvoiceIdContract(
+        string? invoiceId,
+        bool expectedValid)
+    {
+        var analysis = new InvoiceAnalysis(
+            Summary: "Testanalyse",
+            Findings:
+            [
+                new AnalysisFinding(
+                    LineNumber: 6,
+                    InvoiceId: invoiceId,
+                    RuleId: "INV-004",
+                    Description: "Betrag muss größer als null sein.",
+                    NextCheck: "Originalbetrag anhand des Belegs prüfen.",
+                    MissingInformation: ["Originalbetrag"])
+            ],
+            Limitations: []);
+
+        var errors = InvoiceAnalysisValidator.Validate(analysis);
+
+        if (expectedValid)
+            Assert.Empty(errors);
+        else
+            Assert.Single(errors);
+    }
+}