Which issue does this PR close?

• Closes Add primary keys to TPC-H, TPC-DS schemas #22595.

Rationale for this change

The TPC-DS and TPC-H specifications define primary keys, but we previously did not include those constraints when defining the TPC-DS and TPC-H schemas. Including the constraints enables the query optimizer to generate better plans (e.g., by leveraging FDs); it also makes the benchmark setup closer to a realistic TPC-DS/H benchmark run.

To enable this, we need to fix MemTable::load: MemTable could be constructed with_constraints, but those constraints were not attached to the table returned by MemTable::load.

There is some duplication here: we define two copies of the primary keys of both TPC-DS and TPC-H, because benchmarks and test-utils can't easily share code. This could be improved but I'll defer that for now.

What changes are included in this PR?

• Fix MemTable::load to include constraints on the newly loaded table
• Refactor MemTable::load to use collect_partitioned
• Add unit tests for new MemTable::load behavior
• Add TPC-DS and TPC-H primary keys to benchmarks
• Add TPC-DS and TPC-H primary keys to test-utils
• Add TPC-H primary keys to SLT schema definitions

Are these changes tested?

New tests added for MemTable::load constraint behavior. SLT fixtures updated for change in TPC schemas and plans.

Are there any user-facing changes?

No.