fix: prevent iframe leak in untainted prototype and avoid unnecessary iframe creation

[fasyy612]

Summary

• Adopts upstream rrweb #1770 and #1802
• #1770: Wraps untainted prototype iframe creation in try/finally so the iframe is always removed, even on early return (when contentWindow is null) or exception. Previously these iframes would leak into the DOM.
• #1802: Moves querySelector/querySelectorAll from testableAccessors to testableMethods and switches helpers from getUntaintedAccessor to getUntaintedMethod. These are methods, not property accessors, so the accessor check (getOwnPropertyDescriptor(...).get) always failed, causing a throwaway iframe to be created every time just to get the untainted prototype.

Why

Both fixes are in packages/utils/src/index.ts and affect the same getUntaintedPrototype code path. #1770 prevents DOM pollution from leaked iframes. #1802 avoids unnecessary iframe creation on every querySelector/querySelectorAll call, which is a hot path during recording.

Test plan

• Verify no regressions in recording on pages with patched DOM prototypes (Angular apps)
• Inspect DOM during recording to confirm no orphaned iframes from untainted prototype detection

[pauldambra]

could document.createElement fail?

in which case we'd be better with

let iframeEl: blah or blah = null
try {
    iframeEl = document.createElement('iframe');

very not sure if it's even possible for that to throw though!