fix: skip frozen inputs when bookmarking state

[taekop]

Fixes #4026.

serializeReactiveValues() (used by session$doBookmark()) called impl$get(name) for every reactive value, including ones frozen via freezeReactiveValue(). Reading a frozen value raises a silent shiny.silent.error via reactiveStop(), which propagated out of the serialization loop and aborted bookmarking with "Error bookmarking state".

Since a frozen value is explicitly pending an update (and is expected to be thawed on the next flush), it doesn't have a meaningful value to persist at the moment bookmarking runs. This fix skips frozen keys before they're read, so they're omitted from the bookmarked state rather than causing an error.

Added tests in tests/testthat/test-serializers.R covering that a frozen input is excluded without erroring, and that non-frozen inputs still serialize as before.