chore: add empty array repeat fixture and parent-to-child attribute passing

[janechu]

Pull Request

📖 Description

Add more fixture testing for @microsoft/fast-html:

• Empty array repeat fixture: Add test-element-empty-array to the repeat fixture to test the f-repeat directive with an empty array, including a full add/clear lifecycle (0 → 3 → 0 items).
• Parent-to-child attribute passing: Update the nested-elements fixture so that a category attribute on the parent element is forwarded to each child element via the repeat template binding. A new test verifies both the initial rendered value and runtime propagation when the parent attribute is mutated.
• Missing @attr decorators: Add @attr to item_parent on TestElement and TestElementWithObserverMap in the repeat fixture, since these properties are set as HTML attributes in entry.html.

👩‍💻 Reviewer Notes

• The category attribute is intentionally not included in the child mock data — the child receives it solely through the parent template binding (category="{{category}}"), ensuring the test validates real attribute forwarding.
• The @attr decorator was only added to properties that are used as HTML attributes but were missing the decorator. Properties like title, text, and idx remain owned by prepare() / deepMerge() to avoid dual source-of-truth conflicts.

📑 Test Plan

• All existing tests continue to pass (npm run test -w @microsoft/fast-html — 705 passed).
• New test: repeat directive with an empty array should render no items — verifies 0 items initially, adding 3 items, then clearing back to 0.
• New test: should pass parent attribute to child elements — verifies initial category value of "General" on child elements, then mutates parent category to "Updated" and verifies propagation.

✅ 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.