[4.x] Prevent file download responses from leaking into action returns

[joshhanley]

The Scenario

A Livewire action can return a Laravel file download response. Livewire correctly converts the response into a download effect, but the original response object is also included in the action returns effect.

use Illuminate\Support\Facades\Storage;
use Livewire\Component;

class BugPlayground extends Component
{
    public function downloadFileDownloadWay()
    {
        return Storage::disk('local')->download(
            'livewire-downloads/test.txt',
            'test-download-way.txt',
            [
                'Content-Type' => 'text/plain',
            ]
        );
    }
}

The Problem

SupportFileDownloads detects StreamedResponse and BinaryFileResponse instances and converts them into a JSON-safe download effect.

However, HandleComponents::callMethods() still stores the original action return value in effects.returns. For file downloads, that means a Symfony response object can leak into the final Livewire JSON payload.

Returning null from the file-download hook is not enough with the current hook pipeline: component hook finish callbacks are used for side effects, and the event bus treats null as “keep the original forwarded value”.

The Solution

After method call hooks finish, Livewire now normalises StreamedResponse and BinaryFileResponse action returns to null before adding them to effects.returns.

The download effect remains the browser-facing instruction, and the returns array keeps a null entry so action return indexes stay aligned with the submitted calls.

Fixes #10326