Toad does not seem to accept the later session command for use in the explain plan.
alter session set optimizer_index_cost_adj=1;
alter session set optimizer_index_caching = 90;
after these commands, the index is still not chosen though your query is set up with /*+ noparallel(a) */
or /*+ index(a) */ and the statement will chose the index.
Parallel degree is 4 on the pips_item_summary table, there are no indexes on the pips_item_summary_daily table which contains 1 million rows/day, the pips_item_summary table has 250,000,000 rows and is partitioned by date (one week partitions) which enables the parallel execution plan.
Go to View | Options | Oracle | transactions | Uncheck "Execute queries in threads". Then restart Toad.