chore: remove ElementStyles.withBehaviors

[janechu]

Pull Request

📖 Description

Removes ElementStyles.withBehaviors() and the remaining stylesheet
behavior-composition path from @microsoft/fast-element.

This change also:

• removes CSS bindings in css
• removes stylesheet-attached behavior composition from css and css.partial()
• keeps CSSDirective as a static CSS extension point via createCSS(): ComposableStyles
• keeps runtime stylesheet toggling on ElementController.addStyles() / removeStyles()

This also updates the related Playwright coverage, migration guidance,
package docs, and regenerated 3.x API and export-size documentation.

🎫 Issues

• Closes Remove withBehaviors #7244

📑 Test Plan

• npm run build
• npm run prebuild -w sites/website
• npm run biome:check
• npm run checkchange
• npm run test
• npm run test:playwright -w @microsoft/fast-element -- -x --reporter=line

✅ 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.