Escape inline HTML report script sentinels

[graffhyrum]

Fixes #40134

Escape any closing script-tag sentinels before inlining the HTML reporter's report.js bundle into index.html.

This prevents the browser from terminating the inline <script type="module"> element early when the bundle contains \u003c/script sequences.

Regression test: adds a reporter-html test that inspects the generated index.html and verifies the sentinel is escaped before serving the report.

Verification note: I could not run the full Playwright test target in this checkout because the stable test runner dependencies are not installed here.

[graffhyrum]

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