Python: .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.