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

packages/ui/primitives/document-flow/add-fields.tsx

@@ -35,6 +35,7 @@ import { parseMessageDescriptor } from '@documenso/lib/utils/i18n';
 import {
   canRecipientBeModified,
   canRecipientFieldsBeModified,
+  getRecipientsWithMissingFields,
 } from '@documenso/lib/utils/recipients';
 
 import { FieldToolTip } from '../../components/field/field-tooltip';
@@ -555,15 +556,11 @@ export const AddFieldsFormPartial = ({
   };
 
   const handleGoNextClick = () => {
-    const everySignerHasSignature = recipientsByRole.SIGNER.every((signer) =>
-      localFields.some(
-        (field) =>
-          (field.type === FieldType.SIGNATURE || field.type === FieldType.FREE_SIGNATURE) &&
-          field.signerEmail === signer.email,
-      ),
-    );
+    // localFields already have recipientId set correctly (see field creation at line 338)
+    // Using the existing recipientId is important for handling duplicate email recipients
+    const recipientsMissingFields = getRecipientsWithMissingFields(recipients, localFields);
 
-    if (!everySignerHasSignature) {
+    if (recipientsMissingFields.length > 0) {
       setIsMissingSignatureDialogVisible(true);
       return;
     }