tests: add edge-case tests for str_flatten() and str_flatten_comma()

[LeonidasZhak]

Summary

Adds edge-case tests for str_flatten() and str_flatten_comma() covering empty input, single-element input, all-NA input, and NA handling with the last parameter.

Motivation

str_flatten() and str_flatten_comma() are common data cleaning functions for collapsing character vectors into summary strings. The existing tests cover basic usage but miss several edge cases relevant to data cleaning pipelines:

• Empty input with last parameter
• Single-element input with last parameter
• All-NA input with na.rm = TRUE
• str_flatten_comma() with empty input
• str_flatten_comma() with NA values, last, and na.rm

Tests Added (7 test blocks)

1. str_flatten handles empty input with last — Empty character vector with last parameter
2. str_flatten handles single element with last — Single element with last parameter
3. str_flatten handles all-NA input with na.rm — All-NA with na.rm = TRUE
4. str_flatten_comma handles empty input with last — Empty input with last parameter
5. str_flatten_comma handles single element with last — Single element with last
6. str_flatten_comma handles NA with last and na.rm — NA in various positions with last and na.rm

Validation

• Rscript -e 'testthat::test_file("tests/testthat/test-flatten.R")' — 0 failures, 23 passes
• 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.