PLIA-3992: Resolve preview iframe document at check time

[rdom-si]

Summary

• Fixes two related customer-reported bugs on the Optimizely plugin (v4.3.3): the "Run content check" button intermittently missing from the overlay, and prepublish/spellcheck not picking up content or scoring pages.
• Root cause: siteimprove.js captured the preview iframe's document once during initialize() (via a 10s polling loop), then reused that captured reference in the onHighlight and registerPrepublishCallback closures. Any in-CMS navigation after that point replaces the iframe's document, so the callbacks kept pointing at a stale/blank document - explaining both the missing button (polling loop gave up before the iframe existed) and the empty/scoreless checks (checks ran against a blank or detached document).
• Fix: added getPreviewDom(), which resolves the iframe's current document at call time instead of caching it once. Callbacks are now registered immediately after the IsAuthorized check, and the polling loop is removed since there's nothing to wait for anymore.
• Bumped package version 4.3.3 -> 4.3.4 and updated the release changelog.
• Checked in the rebuilt 4.3.4 .nupkg at the repo root, replacing 4.3.3, per this repo's existing release convention.

Test plan

• dotnet build (Debug and Release) succeeds with no errors.
• Verified the packaged module zip (produced by the ZipPluginContent build target) contains the updated siteimprove.js with no remaining references to the removed polling loop.
• Manual verification against a live Optimizely CMS v12 site (e.g. the demo site referenced in PLIA-3992): confirm the "Run content check" button appears without a refresh after navigating between pages, and that a prepublish check on a page with deliberate typos shows the spelling errors and a score.