Add testing guidance on snapshots and waitFor to AGENTS.md

[Eugene (OEvgeny)]

Changelog Entry

Not required, development only.

Description

Two testing practices that came up repeatedly in recent reviews are not in AGENTS.md, so agents keep missing them:

• Asserting computed CSS for visual changes instead of relying on snapshots (review comments on Show focus indicator on suggested actions flipper buttons #5878 and Fix contrast of keyboard focus indicator on card action buttons #5879: "let VRT do the work").
• Checking the effect of asynchronous work right after an action without waiting, which made tests flaky (e.g. Fix flaky hideKeyboard.pauseBetweenSetAttributeAndFocus test #5884).

This PR adds one bullet for each to "Testing instructions".

Specific Changes

• AGENTS.md: two bullets under "Testing instructions": let snapshots verify visual changes and assert state instead of computed CSS; wait for asynchronous effects with waitFor() from @testduet/wait-for

• I have added tests and executed them locally

• I have updated CHANGELOG.md

• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[Copilot]

🟢 Approval recommended

Both additions match the referenced testing guidance and change no runtime behavior.

No open issues 🎉

What changed in this PR

Documents two testing practices in AGENTS.md to improve visual checks and reduce flaky asynchronous assertions.

Changes:

• Prefer snapshots for visual verification and assertions for state.
• Use waitFor() with expect for asynchronous effects instead of sleeps.

File	Description
AGENTS.md	Adds snapshot and asynchronous assertion guidance.

_{Review effort: Balanced}