fix: default chart source to last statement execution with results

[kgopal492]

Summary

Problem (Why)

When editing a chart cell in a DataDoc whose query execution contains multiple statements, the chart composer (useChartSource) defaulted the selected statement to statementIdList[0] — the first statement. If that first statement produced no result rows (a non-result statement like SET/CREATE, or a SELECT returning 0 rows), the chart had no data to bind to and the chart config failed to load in the edit view.

Solution (What + How)

Default the selected statement to the last statement execution that is DONE with result_row_count > 0, rather than blindly picking the first statement. The statementExecutions memo (already computing exactly this filtered list) is moved above the default-selection effect and reused, and a statementExecution && guard is added to the filter to avoid dereferencing missing entries.

Test Plan

Manual verification in a local DataDoc:

1. Run a query cell whose execution has a leading no-result statement and a trailing result statement, e.g.:

   SELECT 1 WHERE false;   -- statement 0: DONE, 0 rows
   SELECT 1 AS a, 2 AS b;  -- last statement: has rows

2. Add a chart cell sourced from that execution and open the chart edit view.
3. Before: chart config/data fails to load (defaults to the empty first statement). After: it defaults to the last statement with rows and loads correctly.