.NET: Add Foundry Memory Context Provider (#3522)

* Add Azure AI Foundry Memory Context Provider with unit tests

* Add FoundryMemory integration tests and sample application

* Fix ClearStoredMemoriesAsync to handle 404 gracefully and rename to EnsureStoredMemoriesDeletedAsync

* Refactor FoundryMemory: simplify architecture and add memory store creation

- Remove IFoundryMemoryOperations interface (was only for test mocking)
- Remove AIProjectClientMemoryOperations wrapper class
- Provider now directly uses AIProjectClient with internal extension methods
- Extension methods return actual response models instead of extracted values
- Remove WaitForUpdateCompletionAsync from provider (sample uses delay)
- Simplify EnsureMemoryStoreCreatedAsync to return Task instead of Task<bool>
- Add memory store creation with chat_model and embedding_model
- Add UpdateMemoriesResponse with SupersededBy and Error fields
- Simplify unit tests to focus on constructor validation and serialization
- Update sample to use simple delay for memory processing wait

* Add waiting operation for memory store updates

* Fix UTF-8 BOM encoding for FoundryMemory csproj files

* Update copilot instructions for UTF-8 BOM and fix sample API rename

* Fix UTF-8 BOM encoding for TestableAIProjectClient.cs

* Add missing response headers for TS

* Changing default embedding

* Using the SDK Models

* Program update

* Remove debugging code from sample

* Adapt FoundryMemoryProvider to new AIContextProvider API and add UTF-8 BOM instruction

- Override ProvideAIContextAsync/StoreAIContextAsync instead of removed virtual InvokingAsync/InvokedAsync
- Use ProviderSessionState<State> for session-scoped state management (matching Mem0Provider pattern)
- Replace constructor-based scope with stateInitializer delegate
- Remove Serialize method (no longer on base class)
- Add SearchInputMessageFilter, StorageInputMessageFilter, StateKey to options
- Update sample to use AIContextProviders list instead of AIContextProviderFactory
- Update unit and integration tests for new API
- Add UTF-8 BOM encoding and --tl:off instructions to dotnet/AGENTS.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Use DefaultAzureCredential in Foundry Memory sample

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review comments for FoundryMemoryProvider

- Move memoryStoreName from options to required constructor parameter
- Make FoundryMemoryProviderScope require non-null/whitespace scope in constructor
- Make Scope property read-only (getter only)
- Replace ConcurrentQueue with single last update ID to fix memory leak
- Only clear pending update ID after successful completion
- Add delete success logging
- Mark FoundryMemoryProvider with [Experimental] attribute
- Update unit tests for new API signatures

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Use Throw.IfNullOrWhitespace for scope and memoryStoreName validation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet/src/Shared/IntegrationTests/FoundryMemoryConfiguration.cs

@@ -0,0 +1,13 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Shared.IntegrationTests;
+
+#pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring as nullable.
+#pragma warning disable CA1812 // Internal class that is apparently never instantiated.
+
+internal sealed class FoundryMemoryConfiguration
+{
+    public string Endpoint { get; set; }
+    public string MemoryStoreName { get; set; }
+    public string? DeploymentName { get; set; }
+}