Add PII redaction POC for secure LLM prompting.

Implements detect-redact-sanitize pipeline with regex, domain rules, and ONNX NER before the LLM boundary, plus NUnit tests and Xenovex push documentation.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/PiiRedaction.Infrastructure.Tests/Onnx/OnnxNerModelRunnerTests.cs

@@ -0,0 +1,53 @@
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using PiiRedaction.Core.Configuration;
+using PiiRedaction.Core.Models;
+using PiiRedaction.Infrastructure.Onnx;
+
+namespace PiiRedaction.Infrastructure.Tests.Onnx;
+
+[TestFixture]
+public sealed class OnnxNerModelRunnerTests
+{
+    [Test]
+    public void Constructor_MissingModel_IsModelUnavailable()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"missing-ner-{Guid.NewGuid():N}.onnx");
+        using var runner = CreateRunner(path);
+
+        runner.IsModelAvailable.Should().BeFalse();
+    }
+
+    [Test]
+    public void PredictEntities_WhenModelUnavailable_ReturnsEmpty()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"missing-ner-{Guid.NewGuid():N}.onnx");
+        using var runner = CreateRunner(path);
+
+        runner.PredictEntities("Customer Ravi Kumar").Should().BeEmpty();
+    }
+
+    [Test]
+    public void Constructor_InvalidModelFile_IsModelUnavailable()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"invalid-ner-{Guid.NewGuid():N}.onnx");
+        File.WriteAllText(path, "not-a-valid-onnx-model");
+
+        try
+        {
+            using var runner = CreateRunner(path);
+            runner.IsModelAvailable.Should().BeFalse();
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+
+    private static OnnxNerModelRunner CreateRunner(string modelPath)
+    {
+        var options = Options.Create(new PiiRedactionOptions { OnnxModelPath = modelPath });
+        return new OnnxNerModelRunner(options, NullLogger<OnnxNerModelRunner>.Instance);
+    }
+}