feat: Thai Tax Withholding Category fixtures with per-company accounts

Add 22 Tax Withholding Category fixtures covering Thailand domestic
withholding (Taw.Paw. 4/2528, Sec. 50(2), Sec. 70) with rates and
thresholds. Fixtures ship without accounts; on import each Thai company
gets "Withholding Tax Payable" (Liability) and "Withholding Tax
Receivable" (Asset) created and the payable account linked. Company
on_update does the same for companies created later. Site-configured
accounts survive fixture re-import on migrate.

default_thai_company/tax_withholding.py

@@ -0,0 +1,104 @@
+import frappe
+
+# Liability: tax we withhold from suppliers and remit on P.N.D.3/53 (linked on every category).
+# Asset: tax customers withhold from us, creditable against corporate income tax.
+LIABILITY_ACCOUNT = "Withholding Tax Payable"
+ASSET_ACCOUNT = "Withholding Tax Receivable"
+
+# Preferred parent groups by root type; first match wins, else the root account.
+PARENT_GROUPS = {
+	"Liability": ("Duties and Taxes", "Current Liabilities"),
+	"Asset": ("Tax Assets", "Current Assets"),
+}
+
+CATEGORY_FILTERS = {"name": ("like", "WHT %")}
+
+
+def thai_companies():
+	return frappe.get_all("Company", filters={"country": "Thailand"}, pluck="name")
+
+
+def get_or_create_account(company, account_name, root_type):
+	name = frappe.db.get_value(
+		"Account", {"company": company, "account_name": account_name, "is_group": 0, "root_type": root_type}
+	)
+	if name:
+		return name
+
+	account = frappe.get_doc(
+		{
+			"doctype": "Account",
+			"company": company,
+			"account_name": account_name,
+			"parent_account": find_parent_group(company, root_type),
+			"root_type": root_type,
+			"account_type": "Tax",
+			"is_group": 0,
+		}
+	).insert(ignore_permissions=True)
+	return account.name
+
+
+def find_parent_group(company, root_type):
+	for account_name in PARENT_GROUPS[root_type]:
+		parent = frappe.db.get_value(
+			"Account",
+			{"company": company, "account_name": account_name, "is_group": 1, "root_type": root_type},
+		)
+		if parent:
+			return parent
+
+	return frappe.db.get_value(
+		"Account",
+		{"company": company, "root_type": root_type, "is_group": 1, "parent_account": ("is", "not set")},
+	)
+
+
+def ensure_company_accounts(company):
+	"""Create both withholding accounts for `company`; return the liability account name."""
+	get_or_create_account(company, ASSET_ACCOUNT, "Asset")
+	return get_or_create_account(company, LIABILITY_ACCOUNT, "Liability")
+
+
+def link_company(category, company, account):
+	"""Append `account` for `company` to the category's accounts table if missing."""
+	if any(row.company == company for row in category.accounts):
+		return False
+	category.append("accounts", {"company": company, "account": account})
+	return True
+
+
+def prepare_fixture_accounts(doc, method=None):
+	"""Tax Withholding Category fixtures ship without `accounts` (company-specific).
+
+	Fixture import re-inserts the doc on every migrate, so: carry over the rows
+	already configured on this site, then link every Thai company, creating the
+	withholding accounts on first use. Mandatory is relaxed for sites without a
+	Thai company yet (setup wizard not run); `setup_company` links them later.
+	"""
+	doc.flags.ignore_mandatory = True
+
+	if not doc.accounts and frappe.db.exists(doc.doctype, doc.name):
+		for row in frappe.get_all(
+			"Tax Withholding Account",
+			filters={"parent": doc.name, "parenttype": doc.doctype},
+			fields=["company", "account"],
+			order_by="idx",
+		):
+			doc.append("accounts", row)
+
+	for company in thai_companies():
+		if not any(row.company == company for row in doc.accounts):
+			link_company(doc, company, ensure_company_accounts(company))
+
+
+def setup_company(doc, method=None):
+	"""Company.on_update: create withholding accounts and link every WHT category."""
+	if doc.country != "Thailand" or not frappe.db.exists("Account", {"company": doc.name}):
+		return
+
+	account = ensure_company_accounts(doc.name)
+	for name in frappe.get_all("Tax Withholding Category", filters=CATEGORY_FILTERS, pluck="name"):
+		category = frappe.get_doc("Tax Withholding Category", name)
+		if link_company(category, doc.name, account):
+			category.save(ignore_permissions=True)