.NET: Surface workflow exceptions instead of a JSON serialization error

[Atharva Vichare (atty57)]

Motivation & Context

When a workflow registered as an agent throws, the user never sees the real failure. They see:

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

The streamed update carries a WorkflowErrorEvent whose Data is the original Exception. The generic workflow-event branch in AgentResponseUpdateExtensions serialized that Data directly, and System.Text.Json rejects Exception.TargetSite (a MethodBase). The resulting NotSupportedException replaced the actual exception message.

Description & Review Guide

• What are the major changes?
  The workflow-event branch in AgentResponseUpdateExtensions now special-cases Exception data and serializes exception.Message instead of the exception object. This mirrors how ExecutorFailedEvent is already handled in the same file. Added WorkflowErrorEventStreamingTests covering the error event and a non-exception payload.

• What is the impact of these changes?
  Clients now receive the actual failure message (e.g. Executor 'AggregatorExecutor' cannot send messages of type '...') rather than a serializer error. No API change; only the emitted data value for error events changes.

• What do you want reviewers to focus on?
  The stack trace is deliberately excluded — the message alone goes to clients, consistent with the existing ExecutorFailedEvent handling. Please confirm that is the intended boundary.

Related Issue

Fixes #8012

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.

[Copilot]

🟢 Approval recommended

The focused fix preserves existing behavior for other payloads and directly tests the reported failure.

Pull request overview

Fixes workflow-as-agent error streaming so clients receive the original exception message instead of a JSON serialization failure.

Changes:

• Serializes exception data as its message.
• Adds regression coverage using an exception with a populated TargetSite.

File summaries

File	Description
AgentResponseUpdateExtensions.cs	Safely serializes workflow exceptions.
WorkflowErrorEventStreamingTests.cs	Verifies the original failure message is streamed.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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