test: add test for write_xpt invisibly returning original data unaltered

[LeonidasZhak]

Summary

Adds a test verifying that write_xpt() returns the original data frame invisibly and unaltered, matching the pattern used by write_dta() and write_sav() in their respective test files.

Changes

• tests/testthat/test-haven-sas.R: Added "invisibly returns original data unaltered" test for write_xpt()

Motivation

Both test-haven-stata.R and test-haven-spss.R have tests verifying that their respective write functions (write_dta() and write_sav()) return the input data invisibly and unaltered. This test was missing for write_xpt() in test-haven-sas.R.

The test verifies an important behavior: that write_xpt() doesn't modify the input data frame (e.g., doesn't strip attributes or change values) and returns it invisibly.

Testing

devtools::test(filter = "haven-sas")
# [ FAIL 1 | WARN 1 | SKIP 0 | PASS 71 ]

The single failure is a pre-existing issue with the cols_only deprecation test (line 132) unrelated to this change.

Relation to existing PRs

• PR test: add tests for read_sas format.sas and display_width XPT non-retention #804 tests read_sas format.sas and display_width XPT non-retention — complementary, no overlap
• PR test: add test for format.sas trailing period stripping in XPT roundtrip #802 tests format.sas trailing period stripping in XPT roundtrip — complementary, no overlap

[LeonidasZhak]

Withdrawing this small automated PR while I consolidate an oversized batch of contributions and reduce maintainer review burden. Sorry for the noise, and thank you for maintaining the project.