Add tester cookie clear endpoint

[ChristianPavilonis]

Description

Add a tester-cookie utility endpoint that lets QA and troubleshooting workflows clear the ts-tester marker cookie after it has been set by GET /_ts/set-tester.

The endpoint should:

• Be disabled by default behind the existing [tester_cookie].enabled flag.
• Return 404 Not Found with no Set-Cookie when disabled.
• Return 204 No Content with Cache-Control: no-store, private when enabled.
• Clear ts-tester on [publisher].cookie_domain using a Set-Cookie expiry header.
• Be wired through both the EdgeZero router and legacy Fastly router.
• Include tests and documentation updates.