.NET: Mark constructors of InvokingContext and InvokedContext as experimental

[westey-m]

Motivation and Context

These types are not intended to be constructed by customers (except when testing their own custom implementations).
Keeping the constructors experimental will allow us to evolve these with additional required parameters, improving the consumption experience. The rest of these classes would be considered GA.

Description

• Mark constructors of InvokingContext and InvokedContext as experimental

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

Marks InvokingContext / InvokedContext constructors as experimental to discourage direct construction by consumers while keeping the rest of the APIs GA, allowing future constructor evolution without breaking changes.

Changes:

• Added [Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)] to InvokingContext / InvokedContext constructors.
• Added #pragma warning disable/restore MAAI001 around internal framework usages of those constructors.
• Added required usings for ExperimentalAttribute and diagnostic IDs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/MessageAIContextProvider.cs	Marks MessageAIContextProvider.InvokingContext ctor as experimental and suppresses internal call-site warnings.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProvider.cs	Marks ChatHistoryProvider.InvokingContext/InvokedContext ctors as experimental and suppresses internal call-site warnings.
dotnet/src/Microsoft.Agents.AI.Abstractions/AIContextProvider.cs	Marks AIContextProvider.InvokingContext/InvokedContext ctors as experimental and suppresses internal call-site warnings.