Update AGENTS.md with PR template and test generation requirements (#88417)
## What?
Updates the AGENTS.md documentation with two clarifications for AI
agents working on the codebase.
## Why?
To improve the quality and consistency of AI-generated contributions by:
1. Ensuring PRs follow the repository's standard description format
2. Making it clear that test generation should use the provided tooling
## How?
Two documentation updates:
1. **PR Template Reference**: Added clarification that PR descriptions
must follow the mandatory format from `.github/pull_request_template.md`
2. **Test Generation Requirement**: Added explicit note that generating
tests using `pnpm new-test` is mandatory, making this requirement more
prominent in the documentation.
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Co-authored-by: Niklas Mischkulnig <4586894+mischnic@users.noreply.github.com>