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.Core.Tests/Sanitization/PromptSanitizerTests.cs

@@ -0,0 +1,26 @@
+using FluentAssertions;
+using PiiRedaction.Core.Sanitization;
+using PiiRedaction.Core.Tests.TestSupport;
+
+namespace PiiRedaction.Core.Tests.Sanitization;
+
+[TestFixture]
+public sealed class PromptSanitizerTests
+{
+    [Test]
+    public void Sanitize_NullRequest_ThrowsArgumentNullException()
+    {
+        var sanitizer = ProductionPipelineFactory.Create().Sanitizer;
+        var action = () => sanitizer.Sanitize(null!);
+        action.Should().Throw<ArgumentNullException>();
+    }
+
+    [TestCase("")]
+    [TestCase("   ")]
+    public void Sanitize_WhitespacePrompt_ThrowsArgumentException(string prompt)
+    {
+        var sanitizer = ProductionPipelineFactory.Create().Sanitizer;
+        var action = () => sanitizer.Sanitize(new(prompt));
+        action.Should().Throw<ArgumentException>();
+    }
+}