fix: AttributeError in Bank Statement Import (backport #40494)

[mergify]

This issue is a combination of 2 different bugs.

1. refactor: disallow bank transactions on different currencies #39336 A bug in a recently added validation on Bank Transaction currency
2. A dormant initialization bug in Bank Statement Import

Scenario on which error occur

This 'AttributeError' only shows up if there were any errors during import, which now it does due to [1] above.
Now, when you retry the import, you'll get the AttributeError due to the uninitialized 'payload_count'.

Cause

'Bank Statement Import' inherits 'Data Import', which has 'payload_count' attribute.
But, It looks like 'Bank Statment Import' is not properly initializing 'Data Import' fields[2]. Since, the Importer class heavly relies on the object being of class 'Data Import', any missing attributes of that class causes hard to debug issues like this.

Traceback with variables (most recent call last):
  File "apps/erpnext/erpnext/accounts/doctype/bank_statement_import/bank_statement_import.py", line 159, in start_import
    i.import_data()
      data_import = <BankStatementImport: Bank Statement Import on 2024-03-17 06:36:39.938968>
      bank_account = 'Feynman Acc - HDFC'
      import_file_path = '/files/Bank Transaction.csv'
      google_sheets_url = None
      bank = 'HDFC'
      template_options = '{"column_to_field_map": {"7": "name"}}'
      file = '/files/Bank Transaction.csv'
      import_file = <frappe.core.doctype.data_import.importer.ImportFile object at 0x109663af0>
      data = [['Date', 'Deposit', 'Withdrawal', 'Description', 'Reference Number', 'Bank Account', 'Currency'], ['2024-02-08', '1344', None, None, 'POS Invoice 1', 'Feynman Acc - HDFC', 'INR']]
      i = <frappe.core.doctype.data_import.importer.Importer object at 0x109663370>
  File "apps/frappe/frappe/core/doctype/data_import/importer.py", line 105, in import_data
    if self.data_import.status == "Partial Success" and len(import_log) >= self.data_import.payload_count:
      self = <frappe.core.doctype.data_import.importer.Importer object at 0x109663370>
      payloads = [{'doc': {'naming_series': 'ACC-BTN-.YYYY.-', 'date': datetime.datetime(2024, 2, 8, 0, 0), 'status': 'Pending', 'bank_account': 'Feynman Acc - HDFC', 'company': 'Lisp 2', 'amended_from': None, 'deposit': 1344.0, 'withdrawal': None, 'currency': 'INR', 'description': None, 'reference_number': 'POS Invoice 1', 'transaction_id': None, 'transaction_type': None, 'allocated_amount': None, 'unallocated_amount': None, 'party_type': None, 'party': None, 'bank_party_name': None, 'bank_party_account_number': None, 'bank_party_iban': None, '__unsaved': 1}, 'rows': [<frappe.core.doctype.data_import.importer.Row object at 0x1099ee620>]}]
      warnings = []
      import_log = [{'row_indexes': '[2]', 'success': 1, 'log_index': 0}]
      log_index = 0
builtins.AttributeError: 'BankStatementImport' object has no attribute 'payload_count'

---

This is an automatic backport of pull request #40494 done by [Mergify](https://mergify.com).

[frappe-pr-bot]

🎉 This PR is included in version 15.17.2 🎉

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