.Net: WIP: feat: Define Workflow and Executor APIs

[Jacob Alber (lokitoth)]

Implements the Workflow APIs for .NET

TODOs:

• Executor and Message Handling
• Workflow Builder
• Higher Order Edges
  • Loop
  • Chain
  • Fan In/Out
• Execution on Runtime
• State/Checkpointing (will be in folloup PR)
• AoT Support
  • Factor out explicit handler registration
  • Fix ILTrim issues
  • Fix DynamicCodeExecution issues
  • Source Generator for ConfigureRoutes() (will be in folloup PR, if needed)
• Source Documentation XML Comments
• Address Remaining feedback

[Copilot]

Pull Request Overview

This PR implements the foundational Workflow APIs for .NET, establishing the core architecture for orchestrating message-based executors within workflows. It introduces the executor pattern for handling typed messages, workflow builders for defining execution flow, and higher-order operations like loops, chains, and fan-in/fan-out patterns.

Key changes:

• Core executor and message handling infrastructure with type-safe message routing
• Workflow builder pattern for constructing execution graphs with conditional edges
• Higher-order workflow operations (loops, chains, fan-in/out) for complex orchestration patterns

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
WorkflowBuilderExtensions.cs	Implements higher-order workflow operations and fan-in/out patterns
WorkflowBuilder.cs	Core workflow construction logic with executor tracking and edge management
TypeErasure.cs	Utility for handling generic ValueTask types in reflection scenarios
MessageRouting.cs	Message dispatch system with reflection-based handler binding
MessageHandler.cs	Core interfaces for typed message handling
Message.cs	Message wrapper types with metadata and retry functionality
Executor.cs	Abstract base class for workflow execution units
ExecutionContext.cs	Interface for execution services
DisposableObject.cs	Base class for async disposal pattern
CompletedValueTaskSource.cs	Compatibility utilities for ValueTask across framework versions

Comments suppressed due to low confidence (1)

dotnet/src/Microsoft.Agents.Orchestration/Workflows/WorkflowBuilderExtensions.cs:17

• Missing closing parenthesis for the string interpolation expression. The line should end with ); instead of ";.

 throw new ArgumentNullException(nameof(value), $"Value cannot be null: {paramExpr}";

[Chris (crickman)]

Are unit-tests and samples intended to be included in this change?

[Chris (crickman)]

Should this be its own project independent from the specific orchestrations?