chore(fast-html): use @microsoft/fast-build to build children fixture

[janechu]

Pull Request

📖 Description

Adds the children test fixture to @microsoft/fast-html and generates its index.html using @microsoft/fast-build, consistent with the other fixtures (attribute, binding, event, ref, slotted).

The children fixture covers f-children attribute directives, which bind references to child DOM nodes in a FAST template (e.g. <ul f-children="{listItems}">). Because f-children is client-side only, @microsoft/fast-build strips it from the rendered Declarative Shadow DOM while preserving the data-fe-c hydration binding count so the FAST runtime can allocate the correct number of binding slots.

The fixture also exercises f-repeat inside the f-children-annotated element, verifying that SSR correctly renders the repeated list items (Foo, Bar) from the state while stripping the client-only directive.

🎫 Issues

No open issues directly addressed.

📑 Test Plan

• npm run build:fixtures in packages/fast-html runs cleanly and regenerates children/index.html — f-children is stripped from the shadow DOM output, data-fe-c binding annotation is preserved, and the f-repeat items are fully rendered.
• All other fixture outputs (attribute, binding, event, ref, slotted) are unchanged.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

⏭ Next Steps

• Add a Playwright spec file (children.spec.ts) covering hydration of the f-children fixture to verify client-side child node reference assignment after SSR.