tests: add edge-case tests for str_pad()

[LeonidasZhak]

Summary

Adds edge-case tests for str_pad() covering common data cleaning scenarios that had limited test coverage.

Rationale

str_pad() is a key text formatting function used in data cleaning pipelines (e.g., padding IDs to fixed width, formatting numeric codes, aligning text for display). The existing test file (45 lines, 9 expectations) only covered:

1. Long strings unchanged (random test)
2. Directions (left/right/both) with simple case
3. Chinese character width
4. Recycling rules
5. Names preservation (left side only)

Missing coverage for common data cleaning scenarios:

1. NA value handling — No tests for NA preservation through padding
2. Empty input — Empty string and empty character vector untested
3. Custom pad characters — Only default space tested
4. Width of 0 — Behavior with zero width untested
5. Boundary widths — Width equal to or less than string width untested
6. Vectorised width — Element-wise width processing untested
7. Vectorised pad — Element-wise pad character processing untested
8. Names preservation — Only left side tested; right and both untested

Tests Added (12 test blocks, 33 new expectations)

1. str_pad() handles NA values — Single NA, mixed NA/vector, NA with right/both sides
2. str_pad() handles empty strings — Empty string with all sides, width 0, width 1
3. str_pad() handles empty character(0) — Empty vector with left and right sides
4. str_pad() respects custom pad character — 0, -, ., * with all sides
5. str_pad() handles width of 0 — No padding applied
6. str_pad() does not pad when width equals string width — All sides
7. str_pad() does not pad when width is less than string width — Width 3, 1, 0
8. str_pad() vectorises width — Left and right sides
9. str_pad() vectorises pad — Left and right sides
10. str_pad() preserves names for all sides — Left, right, both
11. str_pad() validates inputs — Invalid side, invalid use_width type

Validation

• Rscript -e 'devtools::test(filter = "pad")' — 0 failures, 42 passes (9 existing + 33 new)
• Full test suite: 2 pre-existing snapshot mismatches in test-sub.R (unrelated), all other tests pass
• No regressions in existing tests

[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.