[BREAKING] Obsoleting ReflectingExecutor in favor of source gen (#3380)

* Initial working version with tests.

* Updates to validate class data once instead of for each handler method. Also updated Diagnostics Ids to format of MAFGENWF{NUM}

* Formatting and trying to fix generation project pack.

* Another atempt at getting the genrators project to build.

* More attempts to fix generator build and pack.

* Fixing file encodings.

* Initail round of cleanup.

* Trying to fix packing.

* Still trying to fix pipeline pack.

* Remove obsolescence markers, sample updates, and docs from generator branch.

This commit separates the generator core functionality from the
deprecation of ReflectingExecutor. The removed changes will be
re-added in a dependent branch (wf-obsolete-reflector).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Mark ReflectingExecutor and IMessageHandler as obsolete.

This commit deprecates the reflection-based handler discovery approach
in favor of the new [MessageHandler] attribute with source generation.

Changes:
- Add [Obsolete] to ReflectingExecutor<T>, IMessageHandler<T>, IMessageHandler<T,R>
- Add #pragma to suppress warnings in internal reflection code
- Update Concurrent sample to use new [MessageHandler] pattern
- Add Directory.Build.props for samples to include generator
- Add documentation files explaining the migration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Obsoleteing Reflector-based workflow code generation in favor of Source Generators and updating some samples to use new pattern.

This commit deprecates the reflection-based handler discovery approach
in favor of the new [MessageHandler] attribute with source generation.

Changes:
- Add [Obsolete] to ReflectingExecutor<T>, IMessageHandler<T>, IMessageHandler<T,R>
- Add #pragma to suppress warnings in internal reflection code
- Update Concurrent sample to use new [MessageHandler] pattern
- Add Directory.Build.props for samples to include generator
- Add documentation files explaining the migration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Cleaning up temporary design and progress files.

---------

Co-authored-by: alliscode <bentho@microsoft.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Chris <66376200+crickman@users.noreply.github.com>

dotnet/samples/GettingStarted/Workflows/Concurrent/Concurrent/Program.cs

@@ -72,8 +72,8 @@ public static class Program
 /// <summary>
 /// Executor that starts the concurrent processing by sending messages to the agents.
 /// </summary>
-internal sealed class ConcurrentStartExecutor() :
-    Executor<string>("ConcurrentStartExecutor")
+internal sealed partial class ConcurrentStartExecutor() :
+    Executor("ConcurrentStartExecutor")
 {
     /// <summary>
     /// Starts the concurrent processing by sending messages to the agents.
@@ -83,7 +83,8 @@ internal sealed class ConcurrentStartExecutor() :
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.
     /// The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A task representing the asynchronous operation</returns>
-    public override async ValueTask HandleAsync(string message, IWorkflowContext context, CancellationToken cancellationToken = default)
+    [MessageHandler]
+    public async ValueTask HandleAsync(string message, IWorkflowContext context, CancellationToken cancellationToken = default)
     {
         // Broadcast the message to all connected agents. Receiving agents will queue
         // the message but will not start processing until they receive a turn token.