feat: validate discount date in payment schedule (backport #44646)

[mergify]

Early payment discount only makes sense if the discount date is earlier than the due date.

---

This is an automatic backport of pull request #44646 done by [Mergify](https://mergify.com).

[ruthra-kumar]

erpnext.accounts.doctype.coupon_code.test_coupon_code.TestCouponCode
   ✔  test_sales_order_with_coupon_code
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/runner/frappe-bench/apps/frappe/frappe/utils/bench_helper.py", line 109, in <module>
    main()
  File "/home/runner/frappe-bench/apps/frappe/frappe/utils/bench_helper.py", line 18, in main
    click.Group(commands=commands)(prog_name="bench")
  File "/home/runner/frappe-bench/env/lib/python3.10/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/runner/frappe-bench/env/lib/python3.10/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/runner/frappe-bench/env/lib/python3.10/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/runner/frappe-bench/env/lib/python3.10/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/runner/frappe-bench/env/lib/python3.10/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/runner/frappe-bench/env/lib/python3.10/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/runner/frappe-bench/env/lib/python3.10/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/runner/frappe-bench/apps/frappe/frappe/commands/__init__.py", line 29, in _func
    ret = f(frappe._dict(ctx.obj), *args, **kwargs)
  File "/home/runner/frappe-bench/apps/frappe/frappe/commands/utils.py", line 857, in run_parallel_tests
    ParallelTestRunner(
  File "/home/runner/frappe-bench/apps/frappe/frappe/parallel_test_runner.py", line 28, in __init__
    self.run_tests()
  File "/home/runner/frappe-bench/apps/frappe/frappe/parallel_test_runner.py", line 62, in run_tests
    self.run_tests_for_file(test_file_info)
  File "/home/runner/frappe-bench/apps/frappe/frappe/parallel_test_runner.py", line 77, in run_tests_for_file
    self.create_test_dependency_records(module, path, filename)
  File "/home/runner/frappe-bench/apps/frappe/frappe/parallel_test_runner.py", line 96, in create_test_dependency_records
    make_test_records(doctype, commit=True)
  File "/home/runner/frappe-bench/apps/frappe/frappe/test_runner.py", line 360, in make_test_records
    make_test_records(options, verbose, force, commit=commit)
  File "/home/runner/frappe-bench/apps/frappe/frappe/test_runner.py", line 360, in make_test_records
    make_test_records(options, verbose, force, commit=commit)
  File "/home/runner/frappe-bench/apps/frappe/frappe/test_runner.py", line 360, in make_test_records
    make_test_records(options, verbose, force, commit=commit)
  [Previous line repeated 8 more times]
  File "/home/runner/frappe-bench/apps/frappe/frappe/test_runner.py", line 361, in make_test_records
    make_test_records_for_doctype(options, verbose, force, commit=commit)
  File "/home/runner/frappe-bench/apps/frappe/frappe/test_runner.py", line 415, in make_test_records_for_doctype
    frappe.local.test_objects[doctype] += make_test_objects(
  File "/home/runner/frappe-bench/apps/frappe/frappe/test_runner.py", line 474, in make_test_objects
    d.insert(ignore_if_duplicate=True)
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 265, in insert
    self.run_before_save_methods()
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 1061, in run_before_save_methods
    self.run_method("validate")
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 931, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 1283, in composer
    return composed(self, method, *args, **kwargs)
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 1265, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 928, in fn
    return method_object(*args, **kwargs)
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/buying/doctype/purchase_order/purchase_order.py", line 57, in validate
    super().validate()
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/controllers/buying_controller.py", line 37, in validate
    super().validate()
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/controllers/subcontracting_controller.py", line 52, in validate
    super().validate()
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/controllers/stock_controller.py", line 44, in validate
    super().validate()
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/controllers/accounts_controller.py", line 190, in validate
    self.validate_all_documents_schedule()
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/controllers/accounts_controller.py", line 440, in validate_all_documents_schedule
    self.validate_non_invoice_documents_schedule()
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/controllers/accounts_controller.py", line 433, in validate_non_invoice_documents_schedule
    self.validate_payment_schedule_dates()
  File "/home/runner/frappe-bench/apps/erpnext/erpnext/controllers/accounts_controller.py", line 2184, in validate_payment_schedule_dates
    d.validate_from_to_dates("discount_date", "due_date")
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 1544, in validate_from_to_dates
    frappe.throw(
  File "/home/runner/frappe-bench/apps/frappe/frappe/__init__.py", line 548, in throw
    msgprint(
  File "/home/runner/frappe-bench/apps/frappe/frappe/__init__.py", line 509, in msgprint
    _raise_exception()
  File "/home/runner/frappe-bench/apps/frappe/frappe/__init__.py", line 455, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.InvalidDates: <strong>Due Date</strong> must be after <strong>Discount Date</strong>