feat(b2b):B2B-4545 Update v2/frontend/companies to include mandatory customerEmail field (#1291)

<!-- Ticket number or summary of work -->
# [B2B-4545]

## What changed?

* Update POST v2/frontend/companies request schema to include new
required field `customerEmail`
* Add validation error example for missing field

## Release notes draft

- Add new required field `customerEmail` to POST v2/frontend/companies

ping @bc-Vince @bc-sylvialin @chris-nowicki 

[B2B-4545]:
https://bigcommercecloud.atlassian.net/browse/B2B-4545?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: bc-akshatkaushik

docs/b2b-edition/specs/storefront/storefront.yaml

@@ -1201,6 +1201,11 @@ paths:
                   example: 17316289569
                   format: int64
                   type: integer
+                customerEmail:
+                  type: string
+                  format: email
+                  description: "The email address of the BigCommerce customer account for the Company account administrator. Must match the email on the customer account identified by `customerId`."
+                  example: admin@example.com
                 customerId:
                   type: string
                 extraFields:
@@ -1223,6 +1228,7 @@ paths:
                 - companyName
                 - companyPhoneNumber
                 - companyEmail
+                - customerEmail
                 - addressLine1
                 - addressLine2
                 - city

docs/b2b-edition/specs/storefront/storefront/company.yaml

@@ -99,6 +99,7 @@ paths:
                 - companyLastName
                 - companyName
                 - companyPhoneNumber
+                - customerEmail
                 - customerId
                 - state
                 - storeHash
@@ -122,6 +123,8 @@ paths:
                     customerId:
                       type: string
                       description: "The ID of the BigCommerce customer account for the Company account administrator."
+                    customerEmail:
+                      $ref: "#/components/schemas/customerEmail"
                     storeHash:
                       type: string
                       description: "The unique store hash for the BigCommerce store."
@@ -147,6 +150,39 @@ paths:
                     properties:
                       companyId:
                         $ref: "#/components/schemas/companyId_INT"
+        "400":
+          description: Bad Request
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  code:
+                    type: number
+                    description: "The application-level error code for the response."
+                  data:
+                    type: object
+                    properties: {}
+                    description: "Response data."
+                  message:
+                    type: string
+                    description: "A message describing the error."
+              examples:
+                Missing Customer Email:
+                  value:
+                    code: 10013
+                    data: {}
+                    message: "customer email is required"
+                Customer Does Not Exist:
+                  value:
+                    code: 10012
+                    data: {}
+                    message: "customer does not exist"
+                Missing Required Fields:
+                  value:
+                    code: 10013
+                    data: {}
+                    message: "PARAMETER VALUE NOT ALLOW NULL"
   /companies/extra-fields/storefront:
     parameters: []
     get:
@@ -1501,6 +1537,11 @@ components:
             companyEmail:
               description: "The primary email address of the company."
               example: "admin@example.com"
+    customerEmail:
+      type: string
+      format: email
+      description: "The email address of the BigCommerce customer account for the Company account administrator. Must match the email on the customer account identified by `customerId`."
+      example: "admin@example.com"
     catalogId:
       type: string
       description: "The ID of the price list assigned to the Company account."