Allow Guardian scoring for required computer-use models (#40967)

## Why

Models that require automatic review previously skipped Guardian v2 risk scoring
entirely, even when Guardian was limited to computer-use tools. That scope cannot
use a low-risk result to approve unrelated tools, so the blanket skip is
unnecessary.

## What changed

- Retain Guardian v2 risk scoring for required-review models in
  `ComputerUseOnly` scope while continuing to skip it in standard scope.
- Preserve strict automatic approval review for Node REPL-backed tools, including
  when classification fails.

## Testing

Added coverage for low-risk, high-risk, and invalid classifications across the
Node REPL and computer-use REPL paths, plus an unrelated MCP tool.

GitOrigin-RevId: ee38fabf82e196df0ae13b4216b5892be9b799cf