Make ordered aggregate spill test deterministic

## Related PR
#24757

## Problem
`ordered_aggregate_spill.slt:166` intermittently fails with `ResourcesExhausted` under its fixed 600 KiB memory limit. The failure involves two concurrent `OrderedFinalAggregateStream` consumers sharing a greedy pool: one holds about 447.5 KiB, the other about 145.4 KiB, then needs another 8.1 KiB while only 7.0 KiB remains.

### Flaky runs

fail - https://github.com/apache/datafusion/actions/runs/34140768577/job/101802015716?pr=24757
pass - https://github.com/apache/datafusion/actions/runs/34140768577?pr=24757

## Why it matters
A flaky SLT obscures real regressions and makes CI unreliable. It may also expose an ordered-final spill lifecycle/accounting defect rather than test-only nondeterminism.

## Invariant / desired behavior
The test must have a reproducible outcome under a fixed environment: either it consistently completes at 600 KiB, or it reliably exposes a defined resource-accounting failure. Test scheduling must not determine the result.

## Proposed direction
First make the failure classification reproducible before changing production behavior:

- Run the exact SLT repeatedly with identical Rust version, target directory, runner parallelism, pool implementation, and partition count.
- Repeat enough times to capture the interleaving-dependent result; record plan stream types and per-consumer peak/current reservations.
- Add a focused physical-plan reproduction with two ordered-final partitions sharing a finite greedy pool. Force one partition to retain a run while the other reaches its spill boundary.
- Assert the intended result, grouped output, spill metrics, and reservation cleanup. Avoid timing-dependent orchestration.

If the focused test proves the spill transition allocates an unreserved materialized state batch or sort/write workspace, file/fix that production accounting defect separately. Do not weaken this test by raising its memory limit.

## Scope
### In
- Deterministic reproduction and classification of the ordered aggregate spill failure.
- Focused finite-pool, multi-partition regression test coverage.
- Repeated verification under failure-like parallelism.

### Out
- Raising `datafusion.runtime.memory_limit` above 600 KiB.
- Changing aggregate metric docs, labels/timers, or `EXPLAIN ANALYZE` formatting.
- Attributing the flake to the docs PR without a controlled parent-versus-HEAD reproduction.

## Acceptance criteria
- [ ] A focused test deterministically exercises two ordered-final consumers at the spill boundary in a shared finite greedy pool.
- [ ] The test's result is stable across repeated runs with fixed configuration.
- [ ] The test asserts correct grouped output, spill behavior, and zero reservations after stream drop.
- [ ] The existing SLT remains at 600 KiB and is repeatedly stable under relevant parallelism.
- [ ] Evidence identifies whether the cause is test scheduling, runner configuration, or a production spill-accounting defect.

## Tests / verification
- Repeat:
`cargo test -q -p datafusion-sqllogictest --test sqllogictests -- ordered_aggregate_spill --nocapture`
- Run the new focused `datafusion-physical-plan` aggregate test repeatedly with its shared-pool configuration.
- Run relevant `datafusion-physical-plan` aggregate tests.