chore(PartialReduceHashAggregateStream): convert to async generators and cleanup

[rluvaton]

Which issue does this PR close?

Related to:

• [Status Update] Simplifing Streams to be more textbook-like and have less state while keeping the same perf #23974

Rationale for this change

Cleanup the code and remove state

What changes are included in this PR?

changed to async generator and remove unneeded code due to that

Are these changes tested?

existing tests

Are there any user-facing changes?

nope

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.03226% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.85%. Comparing base (541caab) to head (d0c2479).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...sical-plan/src/aggregates/partial_reduce_stream.rs	78.68%	5 Missing and 8 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24015    +/-   ##
========================================
  Coverage   80.84%   80.85%            
========================================
  Files        1096     1099     +3     
  Lines      373936   374271   +335     
  Branches   373936   374271   +335     
========================================
+ Hits       302313   302610   +297     
- Misses      53584    53608    +24     
- Partials    18039    18053    +14     

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[2010YOUY01]

Could we delay those cleanup PRs after #22710 is functionally complete?

Now we're in a inconsistent state, and I think it's better to fully remove the legacy implementation sooner, and next we can continue cleaning things up. I estimate it's ~3 PRs away from that, I'll update in the EPIC issue when it's ready.