Improve Office document code hints

[zhexuany]

Summary

• Add model-visible bash_tool guidance for Office files mounted in the execute_code environment.
• Return Office-specific read_file errors for .docx files instead of generic binary-file guidance.
• Add regression coverage for tool context hints and read_file fallback errors.

Test Plan

• cd packages/api && npx jest src/agents/handlers.spec.ts --runInBand
• cd api && npx jest server/services/Files/Code/process.spec.js --runInBand
• cd packages/api && npm run build
• git diff --check