test(sample-controllers): demonstrate messenger interactions

[revisedbytom]

Explanation

Adds focused examples to the sample controller tests showing how to:

• spy on a controller messenger and assert an external action call
• subscribe to a controller event and assert the published state

This keeps production behavior unchanged and addresses #6766.

References

Fixes #6766

Changelog

No changelog entry is needed because this only changes test examples.

Checklist

• Focused sample-controller tests pass (28 tests)
• Modified files pass ESLint and oxfmt checks
• The monorepo and sample-controller package build successfully

Test commands

• corepack yarn workspace @metamask/sample-controllers run jest --no-coverage packages/sample-controllers/src/sample-gas-prices-controller.test.ts packages/sample-controllers/src/sample-petnames-controller.test.ts
• corepack yarn eslint packages/sample-controllers/src/sample-gas-prices-controller.test.ts packages/sample-controllers/src/sample-petnames-controller.test.ts
• corepack yarn oxfmt --check packages/sample-controllers/src/sample-gas-prices-controller.test.ts packages/sample-controllers/src/sample-petnames-controller.test.ts
• corepack yarn build

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