chore: resolve backport conflicts for #58450

payment_request.py: take the incoming #58438 change in full. version-16-hotfix
lacked the subscription-plan population in make_payment_request() (never got
#57494), so the HEAD side of every hunk was empty -> keep the generic,
permission-checked get_subscription_details() and the subscription_plans
population / is_a_subscription derivation.

test_payment_request.py: kept at the version-16-hotfix state. The cherry-pick
conflict spanned ~1150 lines because the stable branch's test file is far
behind develop; taking "theirs" would have dragged in unrelated test history
from many other PRs. #58438's 4 new tests target TestPaymentRequestV2Gateway
and supplier-side subscription helpers that do not exist on version-16-hotfix,
so they are omitted from the backport.

erpnext/accounts/doctype/payment_request/payment_request.py

@@ -711,6 +711,7 @@ def make_payment_request(**args):
 			party_account = get_party_account(party_type, ref_doc.get(party_type.lower()), ref_doc.company)
 			party_account_currency = get_account_currency(party_account)
 
+		subscription_plans = get_subscription_details(ref_doc.doctype, ref_doc.name)
 		pr.update(
 			{
 				"payment_gateway_account": gateway_account.get("name"),
@@ -742,12 +743,24 @@ def make_payment_request(**args):
 					or gateway_account.get("payment_channel", "Email") != "Email"
 				),
 				"phone_number": args.get("phone_number") if args.get("phone_number") else None,
+				"is_a_subscription": 1 if subscription_plans else 0,
 			}
 		)
 
 		if selected_payment_schedules:
 			apply_payment_references(pr, payment_reference)
 
+		if subscription_plans:
+			pr.set(
+				"subscription_plans",
+				[
+					{
+						"plan": row.plan,
+						"qty": row.qty,
+					}
+					for row in subscription_plans
+				],
+			)
 		# Dimensions
 		pr.update(
 			{
@@ -1061,19 +1074,25 @@ def get_dummy_message(doc):
 
 
 @frappe.whitelist()
-def get_subscription_details(reference_doctype, reference_name):
-	if reference_doctype == "Sales Invoice":
-		subscriptions = frappe.db.sql(
-			"""SELECT parent as sub_name FROM `tabSubscription Invoice` WHERE invoice=%s""",
-			reference_name,
-			as_dict=1,
-		)
-		subscription_plans = []
-		for subscription in subscriptions:
-			plans = frappe.get_doc("Subscription", subscription.sub_name).plans
-			for plan in plans:
-				subscription_plans.append(plan)
-		return subscription_plans
+def get_subscription_details(reference_doctype: str, reference_name: str) -> list[dict]:
+	frappe.has_permission(reference_doctype, "read", reference_name, throw=True)
+
+	if not frappe.get_meta(reference_doctype).has_field("subscription"):
+		return []
+
+	subscription = frappe.db.get_value(reference_doctype, reference_name, "subscription")
+
+	if not subscription:
+		return []
+
+	return frappe.get_all(
+		"Subscription Plan Detail",
+		filters={"parent": subscription, "parenttype": "Subscription", "parentfield": "plans"},
+		fields=[
+			"plan",
+			"qty",
+		],
+	)
 
 
 @frappe.whitelist()