feat: add validation to prevent center owners and mentors from being invited to their own centers

- Implemented checks in CenterMentorSchema to block center owners from being invited as mentors.
- Added validation to prevent mentors from being invited to their own centers or centers they already mentor.
- Enhanced error handling to provide clear feedback when invitation conditions are not met.

src/CenterMentor/centermentor.schema.ts

@@ -147,6 +147,20 @@ export class CenterMentorSchema extends PothosSchema {
             if (!userId) {
               throw new Error('User ID is required')
             }
+            // block invite center owner
+            const centerOwner = await prisma.center.findUnique({
+              where: { centerOwnerId: userId },
+            })
+            if (centerOwner) {
+              throw new Error('Center owner cannot be invited as a mentor')
+            }
+            // block invite mentor to own center and owner from another center
+            const centerMentor = await prisma.centerMentor.findUnique({
+              where: { mentorId: userId },
+            })
+            if (centerMentor) {
+              throw new Error('Mentor already has a center')
+            }
             // get centerId by user id
             const center = await prisma.center.findUnique({
               where: { centerOwnerId: userId },