.NET: Add Cosmos DB implementations for ChatMessageStore and CheckpointStore.

[TheovanKraay]

Motivation and Context

This change adds Cosmos DB implementations for Agent Framework's storage abstractions to provide persistent storage options for chat messages and workflow checkpoints.

Partially addresses issue (.NET only): #1390

Description

Added Cosmos DB implementations for three core storage abstractions:

• CosmosChatMessageStore - Implements ChatMessageStore for chat message persistence
• CosmosCheckpointStore - Implements JsonCheckpointStore for workflow state persistence

Added convenience extensions:

• CosmosDBChatExtensions - Extension methods for configuring chat agents with Cosmos DB
• CosmosDBWorkflowExtensions - Factory methods for creating checkpoint stores

Features implemented:

• Connection string and DefaultAzureCredential authentication
• Hierarchical partitioning (/tenantId/userId/sessionId)
• Transactional batch operations with automatic splitting
• Configurable batch sizes and TTL settings
• State serialization/deserialization support

Added corresponding unit tests.

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? No - this is a new feature addition

[Copilot]

Pull Request Overview

This PR adds Azure Cosmos DB integration to the Agent Framework, providing persistent storage for chat messages and workflow checkpoints. The implementation includes both simple and hierarchical partition key support, along with comprehensive test coverage.

Key Changes

• New Cosmos DB implementations for chat message storage (CosmosChatMessageStore) and checkpoint storage (CosmosCheckpointStore)
• Extension methods for easy configuration of Cosmos DB stores
• Support for connection string, managed identity, and CosmosClient-based authentication
• Comprehensive unit tests for all Cosmos DB components

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 33 comments.

Show a summary per file

File	Description
dotnet/Directory.Packages.props	Added package references for Microsoft.Azure.Cosmos (3.53.1) and Newtonsoft.Json (13.0.3)
dotnet/src/Microsoft.Agents.AI.Abstractions/CosmosChatMessageStore.cs	Implements chat message storage with Cosmos DB, supporting both simple and hierarchical partitioning
dotnet/src/Microsoft.Agents.AI.Abstractions/CosmosAgentThread.cs	Abstract base class for agent threads using Cosmos DB for persistence
dotnet/src/Microsoft.Agents.AI/CosmosDBChatExtensions.cs	Extension methods for configuring Cosmos DB message stores in ChatClientAgentOptions
dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/CosmosCheckpointStore.cs	Implements checkpoint storage with Cosmos DB for workflow persistence
dotnet/src/Microsoft.Agents.AI.Workflows/CosmosDBWorkflowExtensions.cs	Extension methods for creating Cosmos DB checkpoint stores
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/CosmosChatMessageStoreTests.cs	Comprehensive tests for CosmosChatMessageStore including hierarchical partitioning
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/CosmosAgentThreadTests.cs	Tests for CosmosAgentThread functionality and message operations
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/CosmosCheckpointStoreTests.cs	Tests for CosmosCheckpointStore covering checkpoint operations and error handling
Multiple .csproj files	Added package references to Azure.Identity, Microsoft.Azure.Cosmos, and Newtonsoft.Json