fix(api): accept submitter details on public draft PUT and submit POST

S3a PR 1 frontend sends public_submitter_name and public_submitter_email
on draft saves (PUT) and final submit (POST /submit), but the matching
SavePublicDraftRequest and SubmitPublicSubmissionRequest did not whitelist
these fields — Laravel's validated() silently stripped them, preventing
mid-form name/email updates from persisting.

Align both form requests with StartPublicDraftRequest to accept the same
submitter fields with identical rules (string, max:150 / email, max:255,
nullable). Controller copies present keys onto the submission model and
saves when dirty, matching standard Laravel update() semantics — missing
keys leave prior values untouched.

Closes the backend gap identified in PR 1 smoke test.

api/app/Http/Requests/Api/V1/FormBuilder/SavePublicDraftRequest.php

@@ -29,6 +29,8 @@ final class SavePublicDraftRequest extends FormRequest
         $base = [
             'values' => ['sometimes', 'array'],
             'first_interacted_at' => ['nullable', 'date'],
+            'public_submitter_name' => ['nullable', 'string', 'max:150'],
+            'public_submitter_email' => ['nullable', 'email', 'max:255'],
         ];
 
         $schema = $this->resolveSchema();