.NET: [Bug]: Exception in workflow, that is added as agent, results in json serialisation error message instead of exception

@peibekwe is already working on this.

Since Sep 2, 2026.

### Description

Have a workflow that throws an exception. Have the workflow registered as an agent.

Call the agent. Instead of the actual exception being logged, it results in a JSON serialisation error towards the user.

This was an issue in Semantic Kernel already: https://github.com/microsoft/semantic-kernel/issues/10708

### Code Sample

```markdown
var workflowAsAgent = builder
.AddWorkflow("super-agent", CreateWorkflowDelegate)
.AddAsAIAgent(); // Now the workflow can be used as an agent
```

### Error Messages / Stack Traces

Internal (swallowed) exception:
```markdown
Exception has occurred: CLR/System.InvalidOperationException
Exception thrown: 'System.InvalidOperationException' in Microsoft.Agents.AI.Workflows.dll: 'Executor 'AggregatorExecutor' cannot send messages of type 'MySolution.Workflow.RawAggregate'.'
at Microsoft.Agents.AI.Workflows.InProc.InProcessRunnerContext.d__30.MoveNext() in /home/vsts/work/1/s/agent-framework/dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcessRunnerContext.cs:line 235
```

What the user sees:
> Serialization and deserialization of 'System.Reflection.MethodBase' instances is not supported. Path: $.TargetSite. (Code: execution_error)

### Package Versions

```xml










```

### .NET Version

.Net 10.0

### Additional Context

_No response_