Build Guardian V2 synchronous review prompts (#40964)

## What changed

- Add a synchronous reviewer prompt builder that combines root authorization,
  trusted user answers, bounded conversation history, parent environment and
  permission context, retained REPL evidence, and the proposed action.
- Select text or image REPL evidence from Guardian feature and transcript
  settings while respecting the reviewer model's supported modalities and
  shared image limits.
- Render network access requests with trigger-specific guidance and bound
  approval reasons and nested action strings before including them.

## Testing

- Add a prompt test covering root authorization, untrusted transcript framing,
  retry reasons, denied-read restrictions, and action serialization.

GitOrigin-RevId: d1cf886c2e30f7abbf94cce977cef14439ef3f2e