Array.from(str) returns [] for any string containing a lone surrogate #9431

Array.from("a\ud83db")   // node: 3 elements   perry: []
[..."a\ud83db"]          // correct in both

js_array_from_string_codepoints (array/alloc.rs:359) does str::from_utf8(bytes) and returns an empty array on Err — but perry's string payloads are WTF-8, so any lone surrogate makes the whole conversion silently yield nothing. The spread and for…of paths on the same string are correct, which is what makes this a wrong answer rather than a consistent limitation.

Silent: no error, just an empty array where the program expects characters.

Fix note

The walk should use wtf8_step like the other iterators. It was left out of #9427 because that same function holds a raw elements pointer and a borrow of the source payload across per-element allocations — pre-existing, but it means the change wants the RuntimeHandleScope discipline string/split.rs uses rather than a one-line swap.

Found while fixing #9409.