Visualise Agent SDK errors

[tomkis]

Is your feature request related to a problem? Please describe.
We want to visualize errors coming from the upcoming A2A Errors extension. This extension is not yet implemented (#1588 ). Without proper visualization, it's difficult for users to understand what went wrong during A2A operations.

Describe the solution you'd like
Implement standard error toasts to display error messages originating from the A2A Errors extension. The toast should always display a message (mandatory) and optionally include:

• Title (usually the exception name)
• Context (extra fields or code)
• Stacktrace (the stack trace of the error)

All optional fields should support code formatting for better readability.

Additional context
This implementation should be ready to integrate once the A2A Errors extension is available. The design should ensure consistency with existing error handling patterns.

[tomkis]

Check #1588 for the extension proposal.