Deduplicate the TPC-H table schema across scheduler tests and benchmarks #2603

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The ~90-line TPC-H table schema match exists as three byte-identical copies:

• get_tpch_schema in ballista/scheduler/src/test_utils.rs#L154-L250
• tpch_schema in ballista/scheduler/tests/tpch_plan_stability/fixtures.rs#L55-L140
• get_schema in benchmarks/src/bin/tpch.rs#L1149-L1245

A schema change (for example a decimal precision change) has to be made in all three, and nothing checks that they stay the same.

Describe the solution you'd like

Expose a single tpch_schema(table) from one shared location that the scheduler tests and the benchmarks can both depend on, e.g. a ballista_core::test_utils::tpch module, or reuse an existing DataFusion TPC-H helper if one fits. Delete the three copies, and have get_tbl_tpch_table_schema derive from the shared function.

Describe alternatives you've considered

None.

Additional context

This is small and self-contained, a good first issue. Line references are pinned to commit ea1126c.