test: add unit tests for combine_supp function

[ankonyeni]

This PR adds unit tests for the combine_supp() function in R/supp.R. These tests specifically target input validation and variable names collisions to ensure the function fails gracefully with informative error messages when provided with invalid inputs.

Changes
Added test cases to tests/testthat/test-supp.R to verify:

1. Input Type Validation: Ensures both dataset and supp arguments are data frames.
2. Column Collision Detection: Verifies that the function errors out if a variable name in the supplemental dataset (QNAM) already exists in the parent domain, preventing accidental overwriting of data.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.56%. Comparing base (111fd3b) to head (c497b60).
⚠️ Report is 104 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
+ Coverage   82.19%   83.56%   +1.36%     
==========================================
  Files           7        7              
  Lines         691      803     +112     
==========================================
+ Hits          568      671     +103     
- Misses        123      132       +9     

see 4 files with indirect coverage changes

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