.NET: Executor source gen for workflow executor routing

[alliscode]

This pull request introduces a new Roslyn source generator project for the Microsoft Agent Framework Workflows, enabling compile-time route configuration for executor classes using the [MessageHandler] attribute. It includes the generator implementation, diagnostics, build configuration, and integration into the solution and dependency management.

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Follow-up PRs will update samples and docs to use this new pattern and obsolete the ReflectingExecutor

New Source Generator Project:

• Added new project Microsoft.Agents.AI.Workflows.Generators targeting netstandard2.0, with all necessary build and NuGet packaging settings, and Roslyn dependencies for source generator development. [1] [2]

Source Generator Implementation:

• Implemented the ExecutorRouteGenerator class, a Roslyn incremental source generator that scans for [MessageHandler] methods and generates partial classes to configure workflow routes at compile time.
• Added the SourceBuilder utility to generate the actual C# code for route configuration, including overrides for ConfigureRoutes, ConfigureSentTypes, and ConfigureYieldTypes as needed.

Diagnostics and Analysis:

• Introduced DiagnosticDescriptors to define and register custom diagnostics for common usage errors (e.g., missing parameters, invalid return types, non-partial classes, etc.) in handler methods and executor classes.

Solution and Dependency Integration:

• Registered the new generator project and its corresponding unit test project in the solution file (agent-framework-dotnet.slnx). [1] [2]
• Updated Directory.Packages.props to include Microsoft.CodeAnalysis.Analyzers as a dependency for code analysis support.### Motivation and Context

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 source generation for workflow executors, enabling automatic generation of route configuration code for classes decorated with [MessageHandler] attributes. The implementation includes a Roslyn incremental source generator, supporting attributes, comprehensive test infrastructure, and detailed diagnostic reporting.

Changes:

• Adds Microsoft.Agents.AI.Workflows.Generators project with incremental source generator
• Introduces three new attributes: MessageHandlerAttribute, SendsMessageAttribute, and YieldsMessageAttribute
• Creates comprehensive unit test project with 20+ test cases covering various scenarios

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Microsoft.Agents.AI.Workflows.Generators.csproj	New source generator project targeting netstandard2.0
ExecutorRouteGenerator.cs	Main incremental generator implementation using ForAttributeWithMetadataName
SemanticAnalyzer.cs	Two-phase semantic analysis for method and class validation
SourceBuilder.cs	Code generation for ConfigureRoutes and protocol type methods
DiagnosticDescriptors.cs	Seven diagnostic descriptors for validation errors
Models/*.cs	Value-equatable models for incremental generator caching
MessageHandlerAttribute.cs	Attribute to mark handler methods for route generation
SendsMessageAttribute.cs	Class-level attribute declaring sent message types
YieldsMessageAttribute.cs	Class-level attribute declaring yielded message types
Microsoft.Agents.AI.Workflows.Generators.UnitTests.csproj	Test project configuration
GeneratorTestHelper.cs	Helper utilities for testing source generators
ExecutorRouteGeneratorTests.cs	20+ comprehensive test cases covering all scenarios
Microsoft.Agents.AI.Workflows.csproj	Updated to reference generator as analyzer
agent-framework-dotnet.slnx	Solution file updated with new projects
Directory.Packages.props	Added Microsoft.CodeAnalysis.Analyzers package version
Directory.Build.targets	Build configuration for handling incompatible TFMs
SkipIncompatibleBuild.targets	Targets to skip builds with incompatible frameworks

[Copilot]

Typo in comment: 'associted' should be 'associated'.

Suggested change

	// Step 3: Generate source for valid executors using the associted AnalysisResult.
	// Step 3: Generate source for valid executors using the associated AnalysisResult.