Streaming data from remote functions that relies on custom transport regression

[coryvirok]

Describe the bug

This looks like it's the same bug as #13428 but happens when returning objects from remote functions that rely on a custom transport encoder.

Reproduction

https://stackblitz.com/edit/sveltekit-transport-promise-bug-wd7nvqoj?file=src%2Froutes%2Fget-foo.remote.ts

Logs

Uncaught ReferenceError: app is not defined
    at (index):17:45

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/adapter-auto: ^6.0.0 => 6.1.0 
    @sveltejs/kit: ^2.39.1 => 2.39.1 
    @sveltejs/vite-plugin-svelte: ^5.0.0 => 5.1.1 
    svelte: ^5.25.0 => 5.38.10 
    vite: ^6.2.6 => 6.3.6

Severity

serious, but I can work around it

Additional Information

No response