feat: add native support for Microsoft Agent Framework

[philasmar]

Issue #, if available:
DOTNET-8638

Description of Changes

Integrates Microsoft Agent Framework into AWS.AgentCore so that users get the full MS AF pipeline (middleware, context providers, sessions, tool calling) when building agents deployed to AgentCore Runtime.

What changed

Core library (src/AWS.AgentCore/)

• AgentCoreOptions — Added ChatClient, AgentOptions, ConfigureAgent properties. ModelId is now nullable/optional (no longer throws if unset).
• AddAgentCore() — Refactored to register AIAgent and ChatClientAgent in DI with IChatClient priority resolution: explicit ChatClient > pre-registered DI > ModelId Bedrock fallback. Also registers AgentCoreRuntimeContextProvider.
• New AgentCoreRuntimeContextProvider — An AIContextProvider that makes AgentCore runtime context available in the MS AF pipeline.
• New AgentCoreSessionFactory — Internal helper for creating sessions with AgentCore session IDs stored in the session state bag.
• Added InternalsVisibleTo for the unit test project.

Sample apps (all updated to new pattern)

• MicrosoftAgentFrameworkSample — Demonstrates agent middleware, function-calling middleware, system instructions, and multiple tools configured via AgentOptions and ConfigureAgent.
• StreamingAgent — Uses ChatClientAgent from DI with RunStreamingAsync.
• AnnotationsSample — Uses ChatClientAgent via constructor injection, instance-method tools via ChatClientAgentRunOptions.
• AnnotationsStreamingAgent — Same pattern with streaming.
• NativeAotAnnotations / NativeAotExtensions — Updated to new pattern, NativeAOT compatible.

Tests

• 11 unit tests for DI registration logic (IChatClient priority, agent options forwarding, ConfigureAgent callback, error paths).
• 2 unit tests for AgentCoreRuntimeContextProvider.
• 4 unit tests for AgentCoreSessionFactory.
• 4 new integration tests for middleware verification (weather tool, flight tool, CloudWatch log assertions).
• All existing tests pass (54 unit, 8 source generator).

Design philosophy

Thin layer — we don't wrap MS AF features. Users configure tools, middleware, and context providers through standard MS AF APIs (ChatClientAgentOptions, AsBuilder().Use()). When Microsoft adds new features, our library doesn't need changes to support them.

How to use

builder.AddAgentCore(options =>
{
    options.ModelId = "global.anthropic.claude-opus-4-7";
    options.AgentOptions = new ChatClientAgentOptions
    {
        ChatOptions = new()
        {
            Instructions = "You are a helpful assistant.",
            Tools = [AIFunctionFactory.Create(MyTool)]
        }
    };
    options.ConfigureAgent = agent => agent
        .AsBuilder()
        .Use(runFunc: MyMiddleware, runStreamingFunc: null)
        .Build();
});

Breaking changes

• AgentCoreOptions.ModelId changed from string (default "") to string? (default null). Existing code that sets ModelId continues to work.

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