core: fix test failures in test/core/headers.js

[f4ah6o]

Failing file

• test/core/headers.js

Example failure

• FAIL: Core htmx AJAX headers > should include the HX-Request header
• TypeError: Fake server request processing threw exception: Cannot read properties of undefined (reading 'should')

Repro

1. Ensure no !test/... excludes are active in web-test-runner.config.mjs.
2. Run: pnpm exec wtr --config web-test-runner.config.mjs --files test/core/headers.js --concurrency 1 --static-logging

Goal

• Make this file pass in pnpm test:htmx.

[f4ah6o]

実装未対応

• レスポンスヘッダ（HX-Redirect/HX-Refresh/HX-Location/HX-Replace-Url 等）の処理がありません。

実装案

• request.mbt の XHR 完了時に xhr.getResponseHeader を読み取り、
  • HX-Redirect => htmx.location.href を更新
  • HX-Refresh => location.reload()
  • HX-Location => GET を再発行 + history 更新
  • HX-Replace-Url => history.replaceState
• 履歴復元用 loadHistoryFromServer を htmx._ に公開し、historyRestoreAsHxRequest 設定で HX-Request の付与を切替。