[6.x] Extract form submission logic into SubmitForm action

[duncanmcclean]

This pull request extracts the form submission logic from Statamic's FormController into a reusable SubmitForm action class, allowing third-party developers to use it in Livewire components or custom API endpoints.

Usage

use Statamic\Facades\Form;
use Statamic\Facades\Site;
use Statamic\Forms\SubmitForm;
use Statamic\Exceptions\SilentFormFailureException;
use Illuminate\Validation\ValidationException;

$form = Form::find('contact');

try {
    $submission = app(SubmitForm::class)->submit(
        form: $form,
        data: ['name' => 'John', 'email' => 'john@example.com'],
        files: [], // Optional
        site: Site::current(), // Optional
    );
} catch (ValidationException $e) {
    return back()->withErrors($e->errors());
} catch (SilentFormFailureException $e) {
    // Honeypot triggered or event listener rejected
    // $e->submission() contains the submission data

    return back()->with('success', 'Form submitted successfully!');
}

return back()->with('success', 'Form submitted successfully!');

---

Closes statamic/ideas#1409

[jasonvarga]

We should make the action class injectable and you call submit on it, instead of invoking it.

The submit method should also perform validation. Otherwise it'd be very easy to forget to validate and pass in raw request values.

The validator could still be exposed from that class if you wanted to separately validate, like in a livewire component.

The FrontendFormRequest could be cleaned up to avoid validation, since the action would be doing it. And actually our controller seems to already handle ValidationExceptions (thrown by events) but it could catch the one thrown by submit too.