PLU-388: Postman Max 50 recipients (#825)

## Problem
Postman limits each email to have a maximum of 50 recipients (including
the primary recipient, CC recipients, and BCC recipients), but Plumber
does not implement this check.

## Solution
* Add validation on the list of CC recipient emails

**Improvements**:
- Add validation on the list of CC recipient emails and throw error
before attempting to call the Postman API.

## Before & After Screenshots

**BEFORE**:
NA

**AFTER**:
<img width="865" alt="Screenshot 2024-12-18 at 5 30 00 PM"
src="https://github.com/user-attachments/assets/5f82382e-ba04-4f3b-9c21-da8204b27c7f"
/>


## Tests
- [x] Existing emails can be sent correctly
- [x] Step error thrown when more than 49 recipients included in the CC
field

packages/backend/src/apps/postman/__tests__/common/parameters.test.ts

@@ -2,6 +2,10 @@ import { assert, beforeEach, describe, expect, it } from 'vitest'
 
 import { transactionalEmailSchema } from '../../common/parameters'
 
+function generateEmails(length: number) {
+  return Array.from({ length }, (_, i) => `user${i + 1}@example.com`).join(',')
+}
+
 describe('postman transactional email schema zod validation', () => {
   let validPayload: Record<string, unknown>
 
@@ -155,4 +159,22 @@ describe('postman transactional email schema zod validation', () => {
       expect(result.error?.errors[0].message).toEqual('Invalid CC emails')
     },
   )
+
+  it.each([
+    { recipient: 10, cc: 50 },
+    { recipient: 41, cc: 51 },
+    { recipient: 50, cc: 52 },
+    { recipient: 1, cc: 50 },
+  ])(
+    'should fail if total number of emails in Recipient email(s) and CC recipient email(s) exceeds 50',
+    ({ recipient, cc }) => {
+      validPayload.destinationEmail = generateEmails(recipient)
+      validPayload.destinationEmailCc = generateEmails(cc)
+      const result = transactionalEmailSchema.safeParse(validPayload)
+      assert(result.success === false)
+      expect(result.error?.errors[0].message).toEqual(
+        'The total number of CC recipient emails must not exceed 49',
+      )
+    },
+  )
 })

packages/backend/src/apps/postman/common/parameters.ts

@@ -102,6 +102,14 @@ export const transactionalEmailSchema = z.object({
   destinationEmailCc: z
     .string()
     .transform((value, ctx) => validateEmails(value, ctx, 'Invalid CC emails'))
+    /**
+     * NOTE: Postman limits a maximum of 50 recipients (including primary recipient and CC recipients)
+     * Currently, Plumber sends emails to the main recipient individually,
+     * hence a limit of 49 CC recipients is enforced.
+     */
+    .refine((value) => value.length <= 49, {
+      message: 'The total number of CC recipient emails must not exceed 49',
+    })
     .optional(),
   replyTo: z.preprocess((value) => {
     if (typeof value !== 'string') {