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/Llm/MockLlmPromptServiceTests.cs

@@ -0,0 +1,62 @@
+using System.Runtime.CompilerServices;
+using FluentAssertions;
+using Microsoft.Extensions.AI;
+using PiiRedaction.Infrastructure.Llm;
+
+namespace PiiRedaction.Infrastructure.Tests.Llm;
+
+[TestFixture]
+public sealed class MockLlmPromptServiceTests
+{
+    [Test]
+    public async Task SendPromptAsync_ReturnsAssistantText()
+    {
+        var service = new MockLlmPromptService(new MockChatClient());
+
+        const string prompt = "Sanitized <EMAIL_1> content.";
+        var response = await service.SendPromptAsync(prompt);
+
+        response.Should().Contain("Mock LLM Response");
+        response.Should().Contain($"{prompt.Length} chars");
+    }
+
+    [Test]
+    public async Task SendPromptAsync_UsesUserRoleForOutboundMessage()
+    {
+        var capturingClient = new CapturingChatClient();
+        var service = new MockLlmPromptService(capturingClient);
+
+        await service.SendPromptAsync("Only placeholders <PHONE_1>.");
+
+        capturingClient.LastUserMessage.Should().Be("Only placeholders <PHONE_1>.");
+    }
+
+    private sealed class CapturingChatClient : IChatClient
+    {
+        public string? LastUserMessage { get; private set; }
+
+        public Task<ChatResponse> GetResponseAsync(
+            IEnumerable<ChatMessage> messages,
+            ChatOptions? options = null,
+            CancellationToken cancellationToken = default)
+        {
+            LastUserMessage = messages.LastOrDefault(message => message.Role == ChatRole.User)?.Text;
+            return Task.FromResult(new ChatResponse(new ChatMessage(ChatRole.Assistant, "ok")));
+        }
+
+        public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
+            IEnumerable<ChatMessage> messages,
+            ChatOptions? options = null,
+            [EnumeratorCancellation] CancellationToken cancellationToken = default)
+        {
+            await GetResponseAsync(messages, options, cancellationToken);
+            yield break;
+        }
+
+        public object? GetService(Type serviceType, object? serviceKey = null) => null;
+
+        public void Dispose()
+        {
+        }
+    }
+}