fix: use keyed_window.keys for task lookup in accumulator task manager

[BulkBeing]

fixes: #327

I was able to reproduce the issue consistently with the pipeline mentioned in bug report. I'm running the same pipeline locally for an hour now with this fix, no issues so far and messages are reaching sink.

payload.keys is empty on CLOSE operations since no data is attached, causing "accumulator task not found" errors. keyed_window.keys is the authoritative key identity populated for all operation types. Also updates tests to set keyedWindow.keys independently from payload.keys to match real platform behavior.

Java implementation already uses keys of the keyed window: https://github.com/numaproj/numaflow-java/blob/d08cc313271621bf14dc6c0852a8fa8af7904c2f/src/main/java/io/numaproj/numaflow/accumulator/AccumulatorSupervisorActor.java#L84

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.20%. Comparing base (a66cdda) to head (3368ae4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ow/pynumaflow/accumulator/servicer/task_manager.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #330      +/-   ##
==========================================
+ Coverage   94.17%   94.20%   +0.03%     
==========================================
  Files          66       66              
  Lines        3092     3091       -1     
  Branches      162      162              
==========================================
  Hits         2912     2912              
  Misses        149      149              
+ Partials       31       30       -1     

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