[Python] Fix DataFrame Series.value_counts with bins

[anishmehta24]

Fixes #40503

[self._expr] was inside the lambda in the bins branch of value_counts, so ComputedExpression was created without its args and always raised a TypeError. This moves it out, and builds the proxy from a one-element Series since pandas can't compute bins for the empty proxy. Added a test to frames_test.py and a CHANGES.md entry.

Written with help from Claude.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.72%. Comparing base (76f6e6d) to head (d59b30a).
⚠️ Report is 139 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40505       +/-   ##
=============================================
+ Coverage     56.16%   79.72%   +23.55%     
=============================================
  Files          1124      487      -637     
  Lines        178336    82289    -96047     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65603    -34567     
+ Misses        75645    16686    -58959     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.72% <100.00%> (+0.16%)	⬆️

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[github-actions]

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R: @shunping for label python.

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[shunping]

LGTM! Could you rebase to the master and resolve the conflict? Thanks!

[anishmehta24]

Rebased on master and kept both CHANGES.md entries. Thanks!