tests: add edge-case tests for str_remove() and str_remove_all()

[LeonidasZhak]

Summary

Adds edge-case test coverage for str_remove() and str_remove_all():

• NA handling: Tests that both functions correctly propagate NA values (single and mixed with non-NA)
• Empty strings: Tests behavior with empty string input
• Zero-length vectors: Tests that character() input returns character()
• Factor input: Tests factor-to-character coercion works correctly
• No match: Tests that non-matching patterns return the original string

Motivation

str_remove() and str_remove_all() are thin wrappers around str_replace()/str_replace_all(), but the test file (9 lines) had zero coverage for these common data-cleaning edge cases. The DESCRIPTION claims all functions deal with NAs and zero-length vectors consistently — these tests verify that claim for the remove family.

Tests run

[ FAIL 0 | WARN 0 | SKIP 0 | PASS 15 ]

All 15 remove tests pass (5 new test blocks, 13 new assertions). Full suite has 2 pre-existing snapshot failures (lifecycle wording changes), unrelated to this work.

[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.