fix(vendors): skip assignee validation when assignee hasn't changed

The form sends the existing assigneeId on every PATCH even when
the user only changed a different field (e.g. website). If the
vendor's current assignee is a platform admin, every update fails
with "Cannot assign a platform admin as assignee" — even though
no one is trying to change the assignee.

Only run validateAssigneeNotPlatformAdmin when assigneeId is
actually being changed to a different value.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Marfuen

apps/api/src/vendors/vendors.service.ts

@@ -600,9 +600,13 @@ export class VendorsService {
   ) {
     try {
       // First check if the vendor exists in the organization
-      await this.findById(id, organizationId);
+      const existing = await this.findById(id, organizationId);
 
-      if (updateVendorDto.assigneeId) {
+      // Only validate assignee when it's actually changing
+      if (
+        updateVendorDto.assigneeId &&
+        updateVendorDto.assigneeId !== existing.assigneeId
+      ) {
         await this.validateAssigneeNotPlatformAdmin(updateVendorDto.assigneeId, organizationId);
       }