refactor: use extend_doctype_class instead of override_doctype_class

Author: barredterra

CLAUDE.md

@@ -82,7 +82,7 @@ bench --site [sitename] uninstall-app banking
     - `unpaid_vouchers.py`: Payment Entry/Journal Entry creation
 
 **`banking/overrides/`** - ERPNext DocType customizations
-- `bank_transaction.py`: Enhanced reconciliation validation (CustomBankTransaction)
+- `bank_transaction.py`: Enhanced reconciliation validation (`CustomBankTransaction`, mixed in via `extend_doctype_class`)
 - `bank_account.py`: IBAN validation hooks
 
 **`banking/connectors/`** - External service communication
@@ -174,7 +174,7 @@ Tests run against ERPNext's shipped `_Test Company` (INR). Inherit from `frappe.
 
 ### Hooks Configuration
 Key hooks in `hooks.py`:
-- `override_doctype_class`: Replace Bank Transaction with CustomBankTransaction
+- `extend_doctype_class`: Mix `CustomBankTransaction` into Bank Transaction (preferred over `override_doctype_class` in v16+)
 - `doc_events`: Validation and status change hooks
 - `bank_reconciliation_doctypes`: Register Bank Transaction for reconciliation
 - `get_matching_queries`: Point to custom matching logic

banking/hooks.py

@@ -101,9 +101,11 @@
 
 # DocType Class
 # ---------------
-# Override standard doctype classes
+# Extend standard doctype classes
 
-override_doctype_class = {"Bank Transaction": "banking.overrides.bank_transaction.CustomBankTransaction"}
+extend_doctype_class = {
+	"Bank Transaction": ["banking.overrides.bank_transaction.CustomBankTransaction"],
+}
 
 # Document Events
 # ---------------

banking/overrides/bank_transaction.py

@@ -9,6 +9,15 @@
 
 
 class CustomBankTransaction(BankTransaction):
+	"""Mixin applied to **Bank Transaction** via the `extend_doctype_class` hook.
+
+	Inherits from `BankTransaction` so type checkers and IDEs can resolve all
+	DocType fields and inherited methods. At runtime Frappe builds a class
+	`ExtendedBankTransaction(CustomBankTransaction, BankTransaction)`; the
+	resulting MRO is well-defined (classic diamond) and `super()` calls from
+	this mixin still chain into the original `BankTransaction` methods.
+	"""
+
 	def before_validate(self):
 		"""Normalize imported signs before ERPNext computes fees and balances.