fix(agui): Suppress ReActAgent handshake events in AG-UI converters (#2639)

## Summary

This PR suppresses ReActAgent handshake events from the default AG-UI
event stream so they do not fall through to `RAW` events.

Changes include:
- Register `UserConfirmResultEvent` in `PermissionConfirmEventConverter`
as a no-op.
- Add `ExternalExecutionEventConverter` to no-op
`RequireExternalExecutionEvent` and `ExternalExecutionResultEvent`.
- Register the new external execution converter in the AG-UI converter
registry.

## Checklist

Please check the following items before code is ready to be reviewed.

- [ ]  Code has been formatted with `mvn spotless:apply`
- [ ]  All tests are passing (`mvn test`)
- [ ]  Javadoc comments are complete and follow project conventions
- [ ] Related documentation has been updated (e.g. links, examples,
etc.)
- [ ]  Code is ready for review

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Co-authored-by: Joe Zou <yixian.zou@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>