Fix prep data bug

[dylanhmorris]

Closes #810 and adds a test that would have caught it.

[codecov]

Codecov Report

❌ Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.37%. Comparing base (e84032d) to head (7ac2807).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pipelines/prep_eval_data.py	0.00%	2 Missing ⚠️
pipelines/prep_data.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #811      +/-   ##
==========================================
+ Coverage   41.82%   42.37%   +0.54%     
==========================================
  Files          30       30              
  Lines        2809     2834      +25     
==========================================
+ Hits         1175     1201      +26     
+ Misses       1634     1633       -1     

Flag	Coverage Δ
hewr	37.97% <ø> (ø)
pipelines	37.33% <90.32%> (+1.24%)	⬆️
pyrenew_hew	62.29% <ø> (ø)

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[Copilot]

Pull request overview

This PR fixes a bug in the data preparation pipeline by refactoring duplicated pivot/filter logic into a reusable clean_nssp_data function. The fix ensures that the last_data_date filter is applied before pivoting the data, which resolves issue #810. A comprehensive parametrized test is added to validate the fix and prevent regression.

Key changes:

• Introduces clean_nssp_data function to centralize NSSP data cleaning logic
• Refactors prep_eval_data.py and prep_data.py to use the new function
• Adds parametrized test coverage for clean_nssp_data with various edge cases

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
pipelines/prep_data.py	Adds the new clean_nssp_data function and refactors process_and_save_loc_data to use it
pipelines/prep_eval_data.py	Refactors save_eval_data to use the new clean_nssp_data function
pipelines/tests/test_prep_data.py	Adds comprehensive parametrized test for clean_nssp_data function

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[sbidari]

LGTM!
(accepted one copilot suggestion for typo)