GH-48658: [R] Add test for slice_sample with prop = 1 edge case

[HyukjinKwon]

Rationale for this change

The TODO comment in test-dplyr-slice.R asked whether the edge case where prop = 1 should be explicitly handled. The current implementation already correctly handles this case: when prop = 1, the if (prop < 1)
condition is false, so no filtering occurs and all data is returned.

arrow/r/R/dplyr-slice.R

Line 116 in 80e3986

if (prop < 1) {

The TODO was introduced in commit 80e3986 when implementing slice_sample():

arrow/r/tests/testthat/test-dplyr-slice.R

Line 192 in 80e3986

# TODO: handle edge case where prop = 1, do nothing?

This commit added both the if (prop < 1) behavior and the TODO comment, questioning whether the implicit behavior (doing nothing when prop = 1) was correct.

What changes are included in this PR?

This PR adds a test for edge case of prop=1, and removed the todo comment.

Are these changes tested?

Yes, added unittest.

Are there any user-facing changes?

No, test-only.

• GitHub Issue: [R] Add test for slice_sample with prop = 1 edge case #48658

[github-actions]

⚠️ GitHub issue #48658 has been automatically assigned in GitHub to PR creator.