test: make ensure_requirements tests deterministic

[kumarUjjawal]

Which issue does this PR close?

• Closes CI failing on main: test_filter_over_multi_partition_sort_limit is CPU-count dependent (RepartitionExec(32) vs 16 partitions) #22782.

Rationale for this change

The ensure_requirements plan tests were using ConfigOptions::default().
That makes target_partitions depend on the machine CPU count.

Because of that, the same test could build a different physical plan on a
runner with more CPUs. In the failing case, EnsureRequirements added an
extra RepartitionExec, so the snapshot changed even though the optimizer was
doing the right thing.

What changes are included in this PR?

• Add a small test-only config helper with a fixed target_partitions
• Use that helper in the ensure_requirements test helpers and the tests in
  this module that directly build a config
• Keep the plan snapshots and idempotency checks stable across machines with
  different CPU counts

Are these changes tested?

Yes

Are there any user-facing changes?

No