Refactored duplicated logic in setDT and [,:=] to use a helper function

Author: nipun-gupta-3108

.github/workflows/test-coverage.yaml

@@ -4,6 +4,9 @@ on:
   push:
     branches: [master]
   pull_request:
+    branches:
+      - master
+      - refactor-helper-function-issue-6702  # Ensure this branch exists
 
 name: test-coverage.yaml
 
@@ -21,7 +24,7 @@ jobs:
       - uses: r-lib/actions/setup-r@v2
         with:
           use-public-rspm: true
-          r-version: '4.3' # TODO(r-lib/covr#567): Go back to using r-devel
+          r-version: '4.3' # TODO: Go back to using r-devel
 
       - uses: r-lib/actions/setup-r-dependencies@v2
         with:
@@ -51,4 +54,4 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: coverage-test-failures
-          path: ${{ runner.temp }}/package
+          path: ${{ runner.temp }}/package