.NET: Support a message only AIContextProvider as an AIAgent Decorator

[westey-m]

Motivation and Context

#3958

Description

• Introduce a new MessageAIContextProvider that only supports message context enrichment
• Allow the MessageAIContextProvider to be used to decorate any AIAgent instance

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.

[Copilot]

Pull request overview

This pull request introduces MessageAIContextProvider, a new base class that enables message-only context enrichment for any AIAgent instance, addressing issue #3958. The implementation allows providers to enrich agent messages without requiring support for tools and instructions, making context providers more flexible and widely applicable.

Changes:

• Introduced MessageAIContextProvider abstract base class that inherits from AIContextProvider and focuses solely on message enrichment
• Added MessageAIContextProviderAgent decorator that wraps any AIAgent and invokes a pipeline of MessageAIContextProvider instances
• Converted TextSearchProvider, ChatHistoryMemoryProvider, and Mem0Provider to inherit from MessageAIContextProvider
• Added AIAgentBuilder.Use(MessageAIContextProvider[]) extension method for easy integration
• Enhanced ProviderSessionState constructor validation with proper null and whitespace checks
• Exposed ProvideInputMessageFilter and StoreInputMessageFilter as protected properties in AIContextProvider to support inheritance
• Updated samples to demonstrate the new pattern

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/MessageAIContextProvider.cs	New base class for message-only context providers with InvokingAsync and ProvideMessagesAsync methods
dotnet/src/Microsoft.Agents.AI/MessageAIContextProviderAgent.cs	Decorator implementation that enriches messages via provider pipeline before delegating to inner agent
dotnet/src/Microsoft.Agents.AI.Abstractions/AIContextProvider.cs	Exposed filter functions as protected properties to support derived class access
dotnet/src/Microsoft.Agents.AI.Abstractions/ProviderSessionState{TState}.cs	Added null and whitespace validation to constructor parameters
dotnet/src/Microsoft.Agents.AI/TextSearchProvider.cs	Refactored to inherit from MessageAIContextProvider with proper OnDemand validation
dotnet/src/Microsoft.Agents.AI/Memory/ChatHistoryMemoryProvider.cs	Refactored to inherit from MessageAIContextProvider with proper OnDemand validation
dotnet/src/Microsoft.Agents.AI.Mem0/Mem0Provider.cs	Simplified to inherit from MessageAIContextProvider returning messages directly
dotnet/src/Microsoft.Agents.AI/AIAgentBuilder.cs	Added Use extension method accepting MessageAIContextProvider array
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/MessageAIContextProviderTests.cs	Comprehensive tests for MessageAIContextProvider base class functionality
dotnet/tests/Microsoft.Agents.AI.UnitTests/MessageAIContextProviderAgentTests.cs	Tests for decorator pattern including pipeline behavior and error handling
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/ProviderSessionStateTests.cs	Constructor validation tests
dotnet/tests/Microsoft.Agents.AI.UnitTests/Memory/ChatHistoryMemoryProviderTests.cs	MessageAIContextProvider integration tests for ChatHistoryMemoryProvider
dotnet/tests/Microsoft.Agents.AI.UnitTests/Data/TextSearchProviderTests.cs	MessageAIContextProvider integration tests for TextSearchProvider
dotnet/tests/Microsoft.Agents.AI.Mem0.UnitTests/Mem0ProviderTests.cs	MessageAIContextProvider integration tests for Mem0Provider
dotnet/samples/GettingStarted/Agents/Agent_Step20_AdditionalAIContext/Program.cs	Updated sample to use MessageAIContextProvider pattern
dotnet/samples/GettingStarted/Agents/Agent_Step14_Middleware/Program.cs	Added example demonstrating MessageAIContextProvider middleware
dotnet/samples/GettingStarted/Agents/Agent_Step14_Middleware/README.md	Updated documentation to mention MessageAIContextProvider middleware