[FIX] sale_coupon: reset no more eligible applied coupon to Valid state

Before this commit:
When a coupon_program's coupon was giving a reward on a SO and then the
recompute was removing that reward (eg the SO did not met the requirements
anymore) the coupon would not be reset and would be lost.

Now:
If a coupon's reward got removed for some reason, the coupon got reset to a
valid state (`new`) so it can be used again.

Step to reproduce:
  - Generate a coupon from a coupon program that needs requirements (eg $300)
  - Add that coupon to a SO that has a total over $300
  - Coupon is set as `used` and the reward is granted
  - Remove some products to set the SO total below $300
  - Recompute coupon, the reward is correctly removed but the coupon is still
    set as `used` so it won't be usable again and is basically lost.

Closes #3062

models/sale_order.py

@@ -363,10 +363,18 @@ class SaleOrder(models.Model):
         # delete reward line coming from an archived coupon (it will never be updated/removed when recomputing the order)
         invalid_lines = order.order_line.filtered(lambda line: line.is_reward_line and line.product_id.id not in (applied_programs).mapped('discount_line_product_id').ids)
 
+        # Invalid generated coupon for which we are not eligible anymore ('expired' since it is specific to this SO and we may again met the requirements)
         self.generated_coupon_ids.filtered(lambda coupon: coupon.program_id.discount_line_product_id.id in products_to_remove.ids).write({'state': 'expired'})
+        # Reset applied coupons for which we are not eligible anymore ('valid' so it can be use on another )
+        coupons_to_remove = order.applied_coupon_ids.filtered(lambda coupon: coupon.program_id in programs_to_remove)
+        coupons_to_remove.write({'state': 'new'})
+
+        # Unbind promotion and coupon programs which requirements are not met anymore
         order.no_code_promo_program_ids -= programs_to_remove
         order.code_promo_program_id -= programs_to_remove
-        order.applied_coupon_ids -= order.applied_coupon_ids.filtered(lambda coupon: coupon.program_id in programs_to_remove)
+        order.applied_coupon_ids -= coupons_to_remove
+
+        # Remove their reward lines
         invalid_lines |= order.order_line.filtered(lambda line: line.product_id.id in products_to_remove.ids)
         invalid_lines.unlink()
 

tests/test_program_rules.py

@@ -233,8 +233,10 @@ class TestProgramRules(TestSaleCouponCommon):
         order.recompute_coupon_lines()
 
         self.assertEqual(len(order.order_line.ids), 3, "The order should contains the Product A line, the Product B line and the discount line")
+        self.assertEqual(coupon.state, 'used', "The coupon should be set to Consumed as it has been used")
 
         sol1.product_uom_qty = 2
         order.recompute_coupon_lines()
 
         self.assertEqual(len(order.order_line.ids), 2, "The discount line should have been removed as we don't meet the program requirements")
+        self.assertEqual(coupon.state, 'new', "The coupon should be reset to Valid as it's reward got removed")