.NET: Add Foundry Memory Context Provider [WIP]

[rogerbarreto]

Closes #3673

Motivation and Context

Add Azure AI Foundry Memory Context Provider

Introduces Microsoft.Agents.AI.FoundryMemory, a new context provider package that enables agents to store
and retrieve long-term memories using Azure AI Foundry's managed memory service. The provider
automatically extracts memories from conversations and retrieves relevant memories to augment agent
context on each invocation, following the same pattern established by the Mem0 provider.

Features

• FoundryMemoryProvider implementing AIContextProvider with automatic memory extraction and retrieval
• Scoped memory storage by configurable identifier (e.g., user ID) for multi-tenant scenarios
• EnsureMemoryStoreCreatedAsync() to create memory stores with specified chat and embedding models
• EnsureStoredMemoriesDeletedAsync() to clear memories for a scope
• WhenUpdatesCompletedAsync() to poll pending memory updates in parallel until completion
• Session serialization/deserialization support for state persistence
• Configurable options for max memories, update delay, and context prompt
• Sample application demonstrating end-to-end usage
• Unit tests for constructor validation and serialization

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add [BREAKING]\ prefix to the title of the PR.