[Bug] Plan validation not detecting missing tool result blocks

**Bug Description**
no issues in the plan

**Environment Info**
- Platform: darwin
- Terminal: vscode
- Version: 2.1.178
- Feedback ID: 9b52c1a3-1607-4e75-ab86-40695924f990

**Errors**
```json
[]
```

The report names plan validation but no file, test, reproduction steps, or entry point. Start by reproducing the missing tool-result-block case in the reported Darwin/VS Code environment, then locate the validator and its tests; done means the case is detected instead of returning an empty error list.

索引モデルが issue の本文から書いたものです。