docs(agents): encourage parameterised tests in test guidelines

[jonathanlab]

Problem

Greptile flags "Prefer a parameterised test shape" on essentially every PR. The principle is sound but uncodified, so it shows up as repetitive review noise instead of being a documented convention.

Changes

Codify the parameterised-test preference in the test guidelines:

• Added a bullet to AGENTS.md under Testing.
• Added a Parameterised Tests section with an it.each example to docs/testing.md, including the caveat to keep distinct cases as separate tests.

How did you test this?

Docs-only change — no tests run.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code from a Slack thread

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 78a8352.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "docs(agents): encourage parameterised te..." | Re-trigger Greptile}