.NET: Implement Purview middleware in dotnet (#1949)

* Move Purview integration logic into middleware

* Improve error handling and user id management

* Rename purview package

* Handle 402s more explicitly; add Middleware generation methods; don't ignore exceptions

* Use DI container; pass scope id to PC

* Add protection scope caching

* Wrap more exceptions in PurviewClient

* Remove block check dedup; add tests

* Refactor PurviewWrapper intialization; Add unit tests

* Use different .Use method and add IDisposable stub

* Add background job processing for Purview

* Misc comment cleanup

* Apply copilot comments

* Fix formatting

* Formatting other files to fix pipeline

* Small updates to settings and exceptions

* Add README

* Move Purview sample

* Address review comments and update XML comments

* Newline after namespace

* Move public Purview classes to single namespace; Clean up csproj and slnx

* Commit the renames

* Remove unused openAI dependency

---------

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>

dotnet/src/Microsoft.Agents.AI.Purview/Exceptions/PurviewRateLimitException.cs

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+
+namespace Microsoft.Agents.AI.Purview;
+
+/// <summary>
+/// Exception for rate limit exceeded errors from Purview service.
+/// </summary>
+public class PurviewRateLimitException : PurviewException
+{
+    /// <inheritdoc />
+    public PurviewRateLimitException(string message)
+        : base(message)
+    {
+    }
+
+    /// <inheritdoc />
+    public PurviewRateLimitException() : base()
+    {
+    }
+
+    /// <inheritdoc />
+    public PurviewRateLimitException(string? message, Exception? innerException) : base(message, innerException)
+    {
+    }
+}