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/api/v1/implementation.ts

@@ -1041,12 +1041,7 @@ export const ApiContractV1Implementation = tsr.router(ApiContractV1, {
         },
       };
     } catch (err) {
-      return {
-        status: 500,
-        body: {
-          message: 'An error has occured while sending the document for signing',
-        },
-      };
+      return AppError.toRestAPIError(err);
     }
   }),