In plugins/fs/src/commands.rs write_file_inner, a malformed or non-ASCII
`options` header was dropped with .ok(), so baseDir/append/createNew were
silently ignored and the call became a truncating write to the raw path;
the JSON array body fallback truncated values above 255 and dropped
non-numbers.
The header is parsed with parse_write_file_options: empty, `undefined`
(what fetch sends for JSON.stringify(undefined)) and `null` mean no
options, anything else must parse. Array bodies must only contain bytes.
Unit test for the header parser; e2e spec for writes without options,
which is the case the `undefined` handling keeps working.