chore: mark partial reduce stream as can handle back pressure (spillable)

[rluvaton]

Which issue does this PR close?

N/A

Rationale for this change

While adding fuzz tests for aggregate streams I encounter OOMs that partial reduce takes up all the memory and leave nothing to partial

Same as:

datafusion/datafusion/physical-plan/src/aggregates/grouped_hash_stream.rs

Lines 544 to 546 in a5c809f

	// We interpret 'can spill' as 'can handle memory back pressure'.
	// This value needs to be set to true for the default memory pool implementations
	// to ensure fair application of back pressure amongst the memory consumers.

What changes are included in this PR?

mark reduce hash aggregate stream as spillable

What is the testing strategy for this PR?

This was found as part of:

• test: add aggregate fuzz tests for all shapes #24881

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.74%. Comparing base (a5c809f) to head (e3b7610).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25075      +/-   ##
==========================================
- Coverage   81.74%   81.74%   -0.01%     
==========================================
  Files        1128     1128              
  Lines      416644   416645       +1     
  Branches   416644   416645       +1     
==========================================
- Hits       340592   340578      -14     
- Misses      55995    56005      +10     
- Partials    20057    20062       +5     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[2010YOUY01]

LGTM, thank you!