test: add test for format.sas trailing period stripping in XPT roundtrip

[LeonidasZhak]

Adds a test that explicitly verifies the trailing period stripping behavior documented in PR #801.

The SAS transport format does not store trailing periods in format names (e.g., "DATE9." becomes "DATE9"). This test confirms that write_xpt() to read_xpt() correctly strips trailing periods from format.sas attributes.

This test complements the existing "can roundtrip format attribute" test, which only uses formats without trailing periods.

Checks run locally:

• devtools::test(filter = "haven-sas") - all 73 tests pass
• Full test suite: 1 pre-existing snapshot failure in test-labelled-pillar.R (Unicode encoding), unrelated to this change

[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.