fix(stat_bin): name the ngroup computed variable correctly

[raashish1601]

stat_bin documents an ngroup computed variable ("number of points in group", added in #462), but result_dataframe creates the column as "ngroup:" with a stray colon. So mapping it fails:

ggplot(df, aes("x", y=after_stat("count / ngroup"), fill="g")) + geom_histogram(binwidth=1)
# PlotnineError: Could not evaluate the 'y' mapping: 'count / ngroup'
# (original error: name 'ngroup' is not defined)

This renames the column to ngroup, which is also the name already listed in stat_bin.CREATES.

Testing: added test_histogram_ngroup, which fails on main and passes with the fix. tests/test_geom_bar_col_histogram.py, tests/test_binning.py, tests/test_stat_calculate_methods.py and tests/test_geom_freqpoly.py pass, ruff is clean. Added a changelog entry under Bug Fixes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.87%. Comparing base (986704a) to head (8124070).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1135   +/-   ##
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  Coverage   88.87%   88.87%           
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  Files         229      229           
  Lines       17290    17290           
  Branches     2203     2203           
=======================================
  Hits        15366    15366           
  Misses       1295     1295           
  Partials      629      629           

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