fix: Shopping cart Exchange rate validation (#27050)

* fix: Shopping cart Exchange rate validation

- Use `get_exchange_rate` to check for price list exchange rate in cart settings
- Move cart exchange rate validation for Price List from hooks to doc event
- Call cart exchange rate validation on PL update only if PL is in cart and currency is changed

* chore: Comment out obsolete test

- Modifying this test means considering extreme edge cases, which seems pointless now

* fix: Remove snippet that got in due to cherry-pick from `develop`

- This snippet is not present in v13-hotfix. Via https://github.com/frappe/erpnext/pull/26520

Co-authored-by: Nabin Hait <nabinhait@gmail.com>

erpnext/hooks.py

@@ -243,7 +243,7 @@ doc_events = {
 		"on_update": ["erpnext.hr.doctype.employee.employee.update_user_permissions",
 			"erpnext.portal.utils.set_default_role"]
 	},
-	("Sales Taxes and Charges Template", 'Price List'): {
+	"Sales Taxes and Charges Template": {
 		"on_update": "erpnext.shopping_cart.doctype.shopping_cart_settings.shopping_cart_settings.validate_cart_settings"
 	},
 	"Website Settings": {