chore: add payments by product backend validation

[tshuli]

Problem

• There is currently no backend validation for payments by product.

Solution

Bug Fixes:

• Adds backend validation for payments by product

Tests

• Create payments by product. Can submit normally.
  • Load the form. Change the payment amount in form definition. Submission should fail.
  • Load the form. Delete the product in form definition and add another product. Submission should fail.
  • Load the form. Change to multi quantity of 2 to 5. Submission with qty 1 should fail.
  • Load the form. Submission with Qty 6 should fail
  • Load the form. Now disable multi quantity. Submission with qty 4 should fail.
• Create payments by amount form. Can submit normally.

[KenLSM]

Need to remove _id since we only want to test rejection from max_qty changed instead of _id

[tshuli]

👍 thanks for catching