fix: align @attr boolean mode with native HTML boolean attribute semantics

[janechu]

Pull Request

📖 Description

Aligns @attr({ mode: "boolean" }) and booleanConverter with native HTML boolean attribute semantics (e.g., disabled, required):

• Attribute → property is presence-based: setAttribute(name, anyString) (including "" and "false") sets the property to true; only removeAttribute(name) makes it false.
• Property → attribute uses Boolean() coercion: any truthy value adds the attribute (with value ""); any falsy value removes it.

The headline behavior change is that <my-element my-bool="false"> now resolves to element.myBool === true, matching <input disabled="false"> still being disabled.

Changes

• booleanConverter.fromView → !!value (standard Boolean() coercion).
• booleanConverter.toView → value ? "" : null (HTML-standard boolean-attribute output; previously "true"/"false").
• AttributeDefinition.onAttributeChangedCallback special-cases mode: "boolean" to call setValue(element, value !== null), ensuring the attribute → property path is presence-based even when the converter would otherwise coerce "" to false.

This is a breaking change included in the 3.x release.

🎫 Issues

Closes #7511

👩‍💻 Reviewer Notes

• The mode: "boolean" reflection path no longer routes through booleanConverter.toView() / booleanConverter.fromView(). These methods still apply to property assignment (el.bool = X runs fromView), explicit calls (booleanConverter.fromView(x)), and booleanConverter paired with mode: "reflect" (which calls toView).
• Custom converters used with mode: "boolean" will receive a boolean (from value !== null) on the attribute → property path rather than the raw attribute string, since boolean mode now means "presence-based" regardless of the converter.
• Migration guidance is documented in both packages/fast-element/MIGRATION.md and sites/website/src/docs/3.x/migration-guide.md.

📑 Test Plan

Added side-by-side parity tests in packages/fast-element/src/components/attributes.pw.spec.ts that exercise both a FAST mode: "boolean" custom element and a native <button disabled> for every false/true case, asserting they report identical state in both reflection directions.

• 96/96 attribute parity tests pass across chromium, firefox, and webkit.
• 176/176 broader @microsoft/fast-element chromium suite passes (no regressions).
• npm run biome:check, npm run checkchange pass.

✅ 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.