docs: add guidance for code block highlight line numbers to AGENTS.md (#88430)
## Summary
Adds a note to AGENTS.md reminding AI agents to carefully count line
numbers when adding `highlight={...}` attributes to documentation code
blocks.
### Changes
- Added "Documentation Code Blocks" subsection under Development
Anti-Patterns in AGENTS.md
- Documents the importance of accounting for empty lines, imports, and
type imports when specifying highlight line numbers