fix(fs): truncate the file when writeFile writes a stream (#3629)

In plugins/fs/guest-js/index.ts writeFile, the stream path opened the
file with {write, create} but no truncate, unlike the buffer path which
truncates unless `append` is set (commands.rs write_file_inner), so
shorter content left stale trailing bytes.

The stream path now passes `truncate: !options?.append`. api-iife.js
regenerated. E2E spec covers overwrite and append with a stream.

plugins/fs/guest-js/index.ts

@@ -1193,7 +1193,9 @@ async function writeFile(
       read: false,
       create: true,
       write: true,
-      ...options
+      ...options,
+      // overwrite the previous contents, like when writing a buffer
+      truncate: !options?.append
     })
     const reader = data.getReader()