Enable Excel XMLA drill-through by implementing MDSCHEMA_ACTIONS and fixing HIERARCHY_ORIGIN null safety

[Copilot]

Excel's "Show Details" (drill-through) on XMLA PivotTables was silently disabled because Mondrian returned no rows for MDSCHEMA_ACTIONS — the discovery request Excel uses to gate the feature. This also adds a null-safety fix for virtual hierarchies in MDSCHEMA_HIERARCHIES.

MDSCHEMA_ACTIONS — populate drill-through actions per cube

MdschemaActionsRowset.populateImpl() was a no-op. Now emits one Drillthrough action row per visible cube:

Column	Value
ACTION_NAME	"Drillthrough"
ACTION_TYPE	256 (MDACTION_TYPE_ROWSET)
COORDINATE_TYPE	1 (MDACTION_COORDINATE_CUBE)
INVOCATION	1 (interactive)
CUBE_SOURCE	1 (cube)

Three previously stubbed-out columns (ACTION_TYPE, INVOCATION, CUBE_SOURCE) are now defined and included in the rowset schema. Full restriction filtering (catalog / schema / cube / action name) is wired up via makeCondition.

HIERARCHY_ORIGIN null safety

MdschemaHierarchiesRowset dereferenced xmlHierarchy.origin without null-checking xmlHierarchy itself — a crash path for virtual hierarchies. Now guarded:

if (xmlHierarchy == null || xmlHierarchy.origin == null
    || xmlHierarchy.origin.isEmpty())
{
    hierarchyOrigin = 1; // MD_ORIGIN_USER_DEFINED
} else {
    try {
        hierarchyOrigin = Integer.parseInt(xmlHierarchy.origin);
    } catch (NumberFormatException e) {
        hierarchyOrigin = 1;
    }
}

Test reference XML updates

• XmlaBasicTest.ref.xml — testMDActions response now expects 7 action rows (one per FoodMart cube) with the full column schema.
• XmlaBasicTest.ref.xml, XmlaExcel2000Test.ref.xml, XmlaExcelXPTest.ref.xml — DISCOVER_SCHEMA_ROWSETS entries for MDSCHEMA_ACTIONS updated to advertise the new restriction columns.

Integration tests

Added to DrillThroughTest:

• testExcelStyleDrillThroughWithColumnsAndRows — exercises the exact MDX pattern Excel generates (WITH calculated members, FILTER on both axes, RETURN clause).
• testDrillThroughWithColumnsAndRowsAxes — baseline check that DRILLTHROUGH with explicit ON COLUMNS / ON ROWS returns rows.

[dronsv]

Closing — MDSCHEMA_ACTIONS support and DRILLTHROUGH_ACTION_NAME constant are already on main (RowsetDefinition.java:4389+). PR is stale; branch will be deleted.