docs(testing): clarify extension integration homes

[racitores]

Description

Clarify the existing MetaMask Extension integration boundaries without introducing a new testing layer or runner.

• Route full UI + real Redux scenarios to test/integration/.
• Route composed MetaMaskController and child-controller wiring to the existing Node tests under app/scripts/.
• Keep isolated controller logic in unit tests and real browser/extension boundaries in E2E.
• Document that the composed-background tests remain on the unit Jest command despite owning an integration boundary.

Type of Change

• New skill
• Skill improvement/update
• Bug fix
• Documentation update
• Other (please describe):

Checklist

• I have read the CONTRIBUTING.md guidelines
• My skill follows the SKILL_TEMPLATE.md format
• I have tested this skill with an AI agent
• My skill does not contain any secrets, private keys, or sensitive data
• I have added appropriate documentation
• My changes don't break existing skills

Testing

• yarn audit:skills (0 errors; pre-existing warnings only)
• yarn test (65 tests passed)
• git diff --check

Additional Context

This deliberately does not add a Mobile-style component-view or controller integration harness. It documents two existing homes within Extension's current three-layer model and prevents controller-composition coverage from defaulting to E2E.

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