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.

packages/api-e2e/test/specs/fs.spec.ts

@@ -99,6 +99,39 @@ describePlugin('fs', () => {
     expect(read).toEqual(bytes)
   })
 
+  it('writeFile with a stream overwrites or appends', async () => {
+    const result = await tauri(async (api, path) => {
+      const baseDir = api.fs.BaseDirectory.AppData
+      const encoder = new TextEncoder()
+      const streamOf = (...chunks: string[]) =>
+        new ReadableStream<Uint8Array>({
+          start(controller) {
+            for (const chunk of chunks) {
+              controller.enqueue(encoder.encode(chunk))
+            }
+            controller.close()
+          }
+        })
+      await api.fs.writeTextFile(path, 'a much longer previous content', {
+        baseDir
+      })
+      await api.fs.writeFile(path, streamOf('short', ' stream'), { baseDir })
+      const overwritten = await api.fs.readTextFile(path, { baseDir })
+      await api.fs.writeFile(path, streamOf(', appended'), {
+        baseDir,
+        append: true
+      })
+      return {
+        overwritten,
+        appended: await api.fs.readTextFile(path, { baseDir })
+      }
+    }, `${dir}/stream.txt`)
+    expect(result).toEqual({
+      overwritten: 'short stream',
+      appended: 'short stream, appended'
+    })
+  })
+
   it('stat, lstat and size describe files and directories', async () => {
     const result = await tauri(async (api, dir) => {
       const baseDir = api.fs.BaseDirectory.AppData