.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/Serialization/PurviewSerializationUtils.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using Microsoft.Agents.AI.Purview.Models.Common;
+using Microsoft.Agents.AI.Purview.Models.Requests;
+using Microsoft.Agents.AI.Purview.Models.Responses;
+
+namespace Microsoft.Agents.AI.Purview.Serialization;
+
+/// <summary>
+/// Source generation context for Purview serialization.
+/// </summary>
+[JsonSerializable(typeof(ProtectionScopesRequest))]
+[JsonSerializable(typeof(ProtectionScopesResponse))]
+[JsonSerializable(typeof(ProcessContentRequest))]
+[JsonSerializable(typeof(ProcessContentResponse))]
+[JsonSerializable(typeof(ContentActivitiesRequest))]
+[JsonSerializable(typeof(ContentActivitiesResponse))]
+[JsonSerializable(typeof(ProtectionScopesCacheKey))]
+internal sealed partial class SourceGenerationContext : JsonSerializerContext;
+
+/// <summary>
+/// Utility class for Purview serialization settings.
+/// </summary>
+internal static class PurviewSerializationUtils
+{
+    /// <summary>
+    /// Serialization settings for Purview.
+    /// </summary>
+    public static JsonSerializerOptions SerializationSettings { get; } = new JsonSerializerOptions
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        PropertyNameCaseInsensitive = true,
+        WriteIndented = false,
+        AllowTrailingCommas = false,
+        DictionaryKeyPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+        TypeInfoResolver = SourceGenerationContext.Default,
+    };
+}