ENH: Add sort_columns parameter to combine_first
- [x] fixed #60427  
- [x] [Tests added and passed] if fixing a bug or adding a new feature
- [x] All [code checks passed]
- [x] Added [type annotations]to new arguments/methods/functions.
- [x] Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.


This PR enhances the combine_first method in pandas.DataFrame by adding a new parameter, sort_columns, which allows users to control whether the result's column order should be sorted lexicographically or preserve the original order of the calling DataFrame (self).

Currently, combine_first always returns a DataFrame with columns sorted in lexicographical order, which may not be desirable for users who want to maintain the column order of the original DataFrame

```

df1 = pd.DataFrame({"B": [1, 2], "A": [3, 4]})
df2 = pd.DataFrame({"A": [5]}, index=[1])

result = df1.combine_first(df2)
# Current behavior:
#     A  B
# 0  3  1
# 1  4  2

```

With the new sort_columns parameter:

Default Behavior (sort_columns=True): Columns remain sorted as before.
New Behavior (sort_columns=False): Columns retain the order from the original DataFrame (self).


```

df1 = pd.DataFrame({"B": [1, 2], "A": [3, 4]})
df2 = pd.DataFrame({"A": [5]}, index=[1])

result = df1.combine_first(df2, sort_columns=False)
# New behavior:
#     B  A
# 0  1  3
# 1  2  4

```

Tests: Added new test cases in pandas/tests/frame/methods/test_combine_first.py to validate:
Default behavior with sort_columns=True.
Column order preservation with sort_columns=False.

Documentation:
Updated the docstring for combine_first with examples showcasing the new parameter.
Added a changelog entry in doc/source/whatsnew/v3.0.0.rst.


This enhancement maintains backward compatibility, as the default behavior (sort_columns=True) remains unchanged. The new parameter provides additional flexibility for users who need control over column order.





Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.