BUG: Fix AttributeError in from_adjlist when weight column is missing

[nimra06]

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2. This pull request is directed to the pysal/master branch.
3. This pull introduces new functionality covered by
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5. The justification for this PR is:

Found a bug in from_adjlist() where it crashes with AttributeError if the weight column doesn't exist in the DataFrame. The code was using getattr() to check for the column but that checks object attributes not DataFrame columns. So when the column is missing, it throws an error before it can create the default weight column.

I replaced the getattr() with weight_col in adjlist.columns which properly checks if the column exists. Now when the weight column is missing, it creates it with value 1 as the docstring says it should. Added a couple tests to make sure this works for both the default weight column and custom column names.

Fixes #669
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.5%. Comparing base (7304239) to head (8745dc8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #859   +/-   ##
=====================================
  Coverage   85.5%   85.5%           
=====================================
  Files        151     151           
  Lines      16092   16105   +13     
=====================================
+ Hits       13758   13773   +15     
+ Misses      2334    2332    -2     

Files with missing lines	Coverage Δ
libpysal/weights/tests/test_adjlist.py	100.0% <100.0%> (ø)
libpysal/weights/weights.py	93.7% <100.0%> (+0.3%)	⬆️

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