fix(web): flush active view and hover heartbeats on page hide/unload

[Felipe Mamud (fmamud)]

Summary

• Emit a final duration heartbeat for active component views and hovers on visibilitychange hidden, pagehide, and beforeunload, before the lifecycle Insights flush runs.
• Add flushActive() to ElementViewObserver and ElementHoverObserver that reuses the existing heartbeat path (same viewId/hoverId, existing threshold/dedupe). No new event type, no isFinal field.
• Expose flushActiveInteractions() on EntryInteractionRuntime and invoke it from the SDK's visibility callback before flushQueues, preserving queue ordering.

Fixes NT-3718.

Test plan

• pnpm --filter @contentful/optimization-web test:unit — 280 tests pass, including new coverage: active view/hover flush past dwell, no emission before dwell, runtime dispatch to running detectors only, flushActiveInteractions running before sendBatchEvents on pagehide.
• pnpm --filter @contentful/optimization-web typecheck
• pnpm lint
• pnpm format:check
• Shared Playwright E2E — new emits a final hover heartbeat when the page becomes hidden while still hovered in lib e2e-web/e2e/tracking.spec.ts — hovers past dwell, flips document.visibilityState to hidden and dispatches visibilitychange, asserts data-hover-duration-ms advances beyond the pre-hidden snapshot. Verified locally against react-web-sdk on chromium; runs across all supported CSR implementations via CI matrix.

[Charles Hudson (phobetron)]

Very nice!