.NET: Adding AgentRunContext to allow accessing agent run info in external downstream components

[westey (westey-m)]

Motivation and Context

#3411

Description

• Adding AgentRunContext
• Setting AgentRunContext automatically on each run in AIAgent.

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 PR introduces AgentRunContext to provide ambient access to agent run information for downstream components like tools and middleware. This addresses issue #3411 regarding ConversationId propagation in Agent-as-a-Tool scenarios by making agent run context (agent, session, messages, options) accessible via AIAgent.CurrentRunContext using AsyncLocal<T>.

Changes:

• Added AgentRunContext class to encapsulate agent run state (agent, session, request messages, run options)
• Modified AIAgent to set CurrentRunContext using AsyncLocal<T> at the start of each run (both streaming and non-streaming)
• Added comprehensive unit tests for AgentRunContext and updated AIAgent tests to verify context propagation
• Added architectural decision record documenting the design rationale

Reviewed changes

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

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRunContext.cs	New sealed class providing read-only access to agent run information
dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgent.cs	Added AsyncLocal field and CurrentRunContext property; modified RunAsync/RunStreamingAsync to set context
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentRunContextTests.cs	Comprehensive unit tests for AgentRunContext constructor validation and property accessors
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AIAgentTests.cs	Added tests verifying CurrentRunContext is set for all RunAsync/RunStreamingAsync overloads; updated TestAgentSession visibility
docs/decisions/0015-agent-run-context.md	Architectural decision record explaining design choices and trade-offs