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/TestSupport/CapturingChatClient.cs

@@ -0,0 +1,38 @@
+using System.Runtime.CompilerServices;
+using Microsoft.Extensions.AI;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public 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;
+
+        var response = new ChatResponse(new ChatMessage(
+            ChatRole.Assistant,
+            $"Captured {LastUserMessage?.Length ?? 0} chars."));
+
+        return Task.FromResult(response);
+    }
+
+    public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
+        IEnumerable<ChatMessage> messages,
+        ChatOptions? options = null,
+        [EnumeratorCancellation] CancellationToken cancellationToken = default)
+    {
+        var response = await GetResponseAsync(messages, options, cancellationToken).ConfigureAwait(false);
+        yield return new ChatResponseUpdate(ChatRole.Assistant, response.Messages.Last().Text);
+    }
+
+    public object? GetService(Type serviceType, object? serviceKey = null) => null;
+
+    public void Dispose()
+    {
+    }
+}