9bcf5840ef6d317468b11e3e1730278920af91dc  Don't consider a Web Extension service worker as fully loaded until registration has been completed

Classified as not security-relevant.
Triage note: Adds ServiceWorkerThread::hasFinishedFiringActivateEvent and polls (isBackgroundServiceWorkerStillActivating) before dispatching a WebExtension runtime message, plus a testing hook. Timing/correctness for extension messaging, not a security bug.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+1.95), uaf_lifetime (+2.2)