refactor(hash-aggr): Let partial-reduce mode aggregation early emit when OOM

[2010YOUY01]

Which issue does this PR close?

Part of #22710

Rationale for this change

See #24486 for the rationale. That discussion covers the issue and three potential solutions: error, early-emit, and spill.

This PR implements the early-emit behavior agreed on there.

What changes are included in this PR?

Key changes:
In datafusion/physical-plan/src/aggregates/partial_reduce_stream.rs

• top comment explains the high-level ideas
• To understand the implementation, start from poll_next() and follow along

Are these changes tested?

UTs (note partial-reduce aggregation can't be planned from SQL, so we can't do sqllogictests here)

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.50355% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.52%. Comparing base (c429919) to head (969bfc9).
⚠️ Report is 142 commits behind head on main.

Files with missing lines	Patch %	Lines
...sical-plan/src/aggregates/partial_reduce_stream.rs	64.35%	34 Missing and 2 partials ⚠️
datafusion/physical-plan/src/aggregates/mod.rs	82.50%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24785      +/-   ##
==========================================
+ Coverage   81.24%   81.52%   +0.28%     
==========================================
  Files        1113     1123      +10     
  Lines      392744   406130   +13386     
  Branches   392744   406130   +13386     
==========================================
+ Hits       319090   331115   +12025     
- Misses      54900    55649     +749     
- Partials    18754    19366     +612     

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