feat(test): add annotations to TestStepInfo

[Copilot]

TestStepInfo lacked a public TypeScript declaration for annotations, despite supporting them at runtime and in reporters.

Changes

• Expose mutable TestStepInfo.annotations.
• Document annotation type, description, and source location.
• Cover TypeScript usage and reporter propagation.

await test.step('validate result', async stepInfo => {
  stepInfo.annotations.push({
    type: 'expected-result',
    description: 'Step passes with no errors.',
  });
});

• Fixes [Feature]: Add arbitrary annotations to test steps #42387