[R] Add test for slice_sample with prop = 1 edge case

[HyukjinKwon]

Describe the enhancement requested

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.

Component(s)

R