Fixed linear regression estimator for categorical treatments

[jmafoster1]

Summary

This PR fixes LinearRegressionEstimator class to work for categorical treatments.

Checklist

Before you mark your PR as ready for review, please ensure you have completed the following.

• Added unit tests for new functionality
• Properly documented any new code
• Updated Sphinx documentation in docs/ and checked for updated configuration/runtime options
• Added a jupyter notebook tutorial in docs/source/tutorials for any new functionality
• Verified local docs build with sphinx-build -W source/ build

[github-actions]

🦙 MegaLinter status: ⚠️ WARNING

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
⚠️ PYTHON	black	32		1	0.98s
✅ PYTHON	pylint	32		0	5.01s

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.06%. Comparing base (78e0c66) to head (b7c8e1e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
- Coverage   98.26%   98.06%   -0.20%     
==========================================
  Files          26       26              
  Lines        1612     1605       -7     
==========================================
- Hits         1584     1574      -10     
- Misses         28       31       +3     

Files with missing lines	Coverage Δ
..._testing/estimation/linear_regression_estimator.py	93.75% <100.00%> (-6.25%)	⬇️

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[jmafoster1]

Ah, classic codecov getting confused when code goes away!

[jmafoster1]

@jwhogg no rush on this - I'm OOO tomorrow. I set the experiment running on this version in editable mode just to see what happens.

[jwhogg]

lgtm