fix(controllers): make trends queries Postgres-valid (SUM(CASE), GROUP BY)

The *_trends reports (Sales/Purchase Order/Invoice, Delivery Note, etc.) built
raw SQL that is invalid on Postgres:

- `SUM(IF(...))` -> `SUM(CASE WHEN ... ELSE NULL END)` (IF is MySQL-only).
- Loose GROUP BY: each based_on `group by` listed only the key column while the
  SELECT also returned name/territory/group/currency columns. Widen the GROUP BY
  to include every selected non-aggregated column so the query is valid on
  Postgres.
- Add a based_on_key (the first group-by column) for the group-by detail
  subqueries, which equate against a single column (a multi-column group_by
  spliced into an equality produced malformed SQL on both engines).

Behaviour note: widening the GROUP BY can split one based-on group into multiple
report rows when the snapshot columns (territory, renamed customer/item) differ
across transactions, vs MariaDB's previous one-arbitrary-row-per-group. Grand
totals are unchanged (calculate_total_row); per-group subtotals become
deterministic partial sums. This is the accepted widen-vs-arbitrary-pick
tradeoff.

Adds a test (no test file existed) running Sales Order Trends with a group_by,
exercising the widened GROUP BY / based_on_key / SUM(CASE) on both engines.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

erpnext/selling/report/sales_order_trends/test_sales_order_trends.py

@@ -0,0 +1,26 @@
+# Copyright (c) 2015, Frappe Technologies Pvt. Ltd. and Contributors
+# License: GNU General Public License v3. See license.txt
+
+from erpnext.tests.utils import ERPNextTestSuite
+
+
+class TestSalesOrderTrends(ERPNextTestSuite):
+	def test_report_executes_with_group_by(self):
+		# trends.get_data builds per-period SUM(CASE ...) aggregates (converted from MySQL SUM(IF)),
+		# with a GROUP BY widened to every selected non-aggregated column and a based_on_key for the
+		# group-by detail subqueries. Setting group_by exercises that full path on both engines.
+		from erpnext.selling.doctype.sales_order.test_sales_order import make_sales_order
+		from erpnext.selling.report.sales_order_trends.sales_order_trends import execute
+
+		make_sales_order(item_code="_Test Item", qty=3, rate=100)
+
+		filters = {
+			"company": "_Test Company",
+			"period": "Monthly",
+			"based_on": "Item",
+			"group_by": "Customer",
+		}
+		columns, data, _chart_none, _chart = execute(filters)
+
+		self.assertTrue(columns)
+		self.assertTrue(any("_Test Item" in [str(cell) for cell in row] for row in data))