Allow setnafill() to accept logical vectors in cols (#4113)

[venom1204]

closes #4113

this pr enables setnafill() to accept logical vectors for the cols argument, allowing for dynamic column selection via conditions like sapply(DT, is.numeric). The implementation updates the internal colnamesInt C utility to handle logical vectors with exact length matching and NA validation while ensuring that existing error-handling logic remains consistent across the package.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (895067d) to head (1794b75).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7841      +/-   ##
==========================================
- Coverage   99.02%   99.01%   -0.01%     
==========================================
  Files          88       88              
  Lines       17266    17280      +14     
==========================================
+ Hits        17097    17110      +13     
- Misses        169      170       +1     

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[github-actions]

• HEAD=issue4113 much faster for transform improved in #5493
  

Generated via commit 1794b75

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	6 minutes and 33 seconds
Installing different package versions	12 minutes and 58 seconds
Running and plotting the test cases	6 minutes and 54 seconds