Fix non-equi join error with equi + 2 inequality conditions (#7641)

[tarun-t]

Summary

Fixes #7641. Non-equi joins combining an equality condition with two inequality conditions on the same column (e.g., on = .(id == id, val >= lo, val <= hi)) error on
1.18.0+ due to rightcols being overwritten by chmatchdup.

When rightcols has duplicates (same x column in multiple conditions), chmatchdup remaps them into the expanded result namespace nx. This overwrote rightcols,
causing downstream code (.shallow(x, rightcols) in .join_result_key) to reference columns beyond ncol(x).

Fix: Introduce ansrightcols for the remapped indices, keep rightcols as original column indices into x. Only icolsAns needs the remapped values; all other
downstream uses need the original x-relative indices.

Changes

• R/data.table.R: Replace rightcols = chmatchdup(...) with ansrightcols = chmatchdup(...), use ansrightcols only for icolsAns
• inst/tests/tests.Rraw: Regression test (1483.91)
• NEWS.md: Bug fix entry

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (1bd88cb) to head (20849ca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7642   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          87       87           
  Lines       16896    16897    +1     
=======================================
+ Hits        16732    16733    +1     
  Misses        164      164           

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