.NET: [BREAKING] Add ChatClient decorator for calling AIContextProviders

[westey-m]

Motivation and Context

#3957

Description

• Add ChatClient decorator and ChatClientBuilder extension to allow calling AIContextProviders

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

Adds decorator-based middleware to invoke AIContextProvider/MessageAIContextProvider pipelines, enabling context enrichment before chat/agent execution and provider notifications after completion, aligning with issue #3957’s request for an IChatClient-level decorator.

Changes:

• Introduces AIContextProviderChatClient and a ChatClientBuilder.Use(...) extension to invoke AIContextProviders around IChatClient calls.
• Introduces MessageAIContextProviderAgent to invoke MessageAIContextProviders around AIAgent runs (including streaming).
• Adds unit tests and updates the Step14 middleware sample/docs to demonstrate the new pipeline.

Reviewed changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/AIContextProviderDecorators/AIContextProviderChatClient.cs	New delegating chat client that invokes AIContextProvider pipeline for non-streaming and streaming calls.
dotnet/src/Microsoft.Agents.AI/AIContextProviderDecorators/AIContextProviderChatClientBuilderExtensions.cs	Adds ChatClientBuilder.Use(params AIContextProvider[]) extension to register the decorator.
dotnet/src/Microsoft.Agents.AI/AIContextProviderDecorators/MessageAIContextProviderAgent.cs	New delegating agent that invokes MessageAIContextProvider pipeline around agent runs and streaming.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AIContextProviderDecorators/AIContextProviderChatClientTests.cs	Unit tests covering enrichment, sequencing, success/failure notifications, and builder integration for chat client decorator.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AIContextProviderDecorators/MessageAIContextProviderAgentTests.cs	Unit tests covering enrichment, sequencing, and success/failure notifications for agent decorator (incl. streaming).
dotnet/samples/GettingStarted/Agents/Agent_Step14_Middleware/Program.cs	Sample updates to demonstrate using AIContextProvider in the IChatClient pipeline.
dotnet/samples/GettingStarted/Agents/Agent_Step14_Middleware/README.md	Documentation update listing the new chat-client-level AIContextProvider middleware example.