[fizz][external-runtime] Fix: process mutation records before disconnecting (#26169)

> All notifications of mutations that have already been detected, but
not yet reported to the observer, are discarded. To hold on to and
handle the detected but unreported mutations, use the takeRecords()
method.
>    -- ([Mozilla docs for disconnect](

https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver/disconnect))

Fizz external runtime needs to process mutation records (representing
potential Fizz instructions) before calling `disconnect()`. We currently
do not do this (and might drop some instructions).