feat: validate signers have signature fields before distribution (#2411)

API users were inadvertently sending documents without signature fields,
causing confusion for recipients and breaking their signing flows.

- Add getRecipientsWithMissingFields helper in recipients.ts
- Add server-side validation in sendDocument to block distribution
- Fix v1 API to return 400 instead of 500 for validation errors
- Consolidate UI signature field checks to use isSignatureFieldType
- Add E2E tests for both v1 and v2 APIs

apps/remix/app/components/general/envelope-signing/envelope-signer-form.tsx

@@ -1,8 +1,9 @@
 import { useMemo } from 'react';
 
 import { Plural, Trans } from '@lingui/react/macro';
-import { FieldType, RecipientRole } from '@prisma/client';
+import { RecipientRole } from '@prisma/client';
 
+import { isSignatureFieldType } from '@documenso/prisma/guards/is-signature-field';
 import { Input } from '@documenso/ui/primitives/input';
 import { Label } from '@documenso/ui/primitives/label';
 import { RadioGroup, RadioGroupItem } from '@documenso/ui/primitives/radio-group';
@@ -30,7 +31,7 @@ export default function EnvelopeSignerForm() {
   const { isNameLocked, isEmailLocked } = useEmbedSigningContext() || {};
 
   const hasSignatureField = useMemo(() => {
-    return recipientFields.some((field) => field.type === FieldType.SIGNATURE);
+    return recipientFields.some((field) => isSignatureFieldType(field.type));
   }, [recipientFields]);
 
   const isSubmitting = false;