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/Integration/LlmBoundaryTests.cs

@@ -0,0 +1,52 @@
+using FluentAssertions;
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+using PiiRedaction.Core.Tests.TestSupport;
+using PiiRedaction.Infrastructure.Llm;
+
+namespace PiiRedaction.Core.Tests.Integration;
+
+[TestFixture]
+public sealed class LlmBoundaryTests
+{
+    [Test]
+    public async Task SendPromptAsync_DoesNotTransmitOriginalPii()
+    {
+        var capturingClient = new CapturingChatClient();
+        ILlmPromptService service = new MockLlmPromptService(capturingClient);
+
+        const string sanitized = "Customer <PERSON_1> with email <EMAIL_1>.";
+        await service.SendPromptAsync(sanitized);
+
+        capturingClient.LastUserMessage.Should().Be(sanitized);
+        capturingClient.LastUserMessage.Should().NotContain("ravi.kumar@gmail.com");
+        capturingClient.LastUserMessage.Should().NotContain("Ravi Kumar");
+    }
+
+    [Test]
+    public async Task SendPromptAsync_AfterSanitization_OnlyPlaceholdersReachLlm()
+    {
+        var capturingClient = new CapturingChatClient();
+        var sanitizer = ProductionPipelineFactory.Create().Sanitizer;
+        ILlmPromptService service = new MockLlmPromptService(capturingClient);
+
+        const string prompt = "Email ravi.kumar@gmail.com please.";
+        var result = sanitizer.Sanitize(new SanitizationRequest(prompt));
+
+        result.SanitizedPrompt.Should().Contain("<EMAIL_1>");
+        await service.SendPromptAsync(result.SanitizedPrompt);
+
+        capturingClient.LastUserMessage.Should().Contain("<EMAIL_1>");
+        capturingClient.LastUserMessage.Should().NotContain("ravi.kumar@gmail.com");
+    }
+
+    [TestCase(null)]
+    [TestCase("")]
+    [TestCase("   ")]
+    public void SendPromptAsync_InvalidSanitizedPrompt_ThrowsArgumentException(string? prompt)
+    {
+        ILlmPromptService service = new MockLlmPromptService(new MockChatClient());
+        var action = async () => await service.SendPromptAsync(prompt!);
+        action.Should().ThrowAsync<ArgumentException>();
+    }
+}