fix(language-server): defer HTML expression completions to TS

[felmonon]

Summary

• let the HTML plugin return null when completion is requested inside a brace expression so the TypeScript service can handle it
• add a regression test covering completions in HTML attribute expressions like href={some}
• add a changeset for the language server fix

Fixes #14671.

Testing

• pnpm -C packages/language-tools/language-server build
• pnpm -C packages/language-tools/language-server exec astro-scripts test test/typescript/completions.test.ts --teardown-test ./test/misc/teardown.ts --tsx true --setup ./test/setup.js
• pnpm -C packages/language-tools/language-server exec astro-scripts test test/html/completions.test.ts --teardown-test ./test/misc/teardown.ts --tsx true --setup ./test/setup.js
• pnpm -C packages/language-tools/language-server exec astro-scripts test test/{html/completions.test.ts,typescript/completions.test.ts} --teardown-test ./test/misc/teardown.ts --tsx true --setup ./test/setup.js

[changeset-bot]

🦋 Changeset detected

Latest commit: 95ab634

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[Princesseuh]

Tried this in local and it seemed to work fine!