tests: add edge-case tests for str_count()

[LeonidasZhak]

Summary

Adds edge-case tests for str_count() covering data cleaning scenarios that were previously untested.

Tests Added

1. NA input handling — str_count(NA, "a") returns NA_integer_; vector with mixed NA propagates correctly
2. Empty string input — str_count("", "a") returns 0L; empty character vector returns integer()
3. NA pattern errors — str_count("abc", NA_character_) errors with informative message
4. Non-overlapping regex matches — Verifies correct counting of non-overlapping regex matches (e.g., "aaa" with "aa" counts 1, "aaaa" counts 2)
5. Multibyte characters — Counting accented characters and CJK characters works correctly

Validation

• All 22 tests pass (was 12 before): devtools::load_all(); test_file('tests/testthat/test-count.R')
• No regressions in existing tests
• No snapshot files needed (uses expect_error() instead of expect_snapshot() for error test)

Files Changed

• tests/testthat/test-count.R — Added 25 lines (5 test blocks)

[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.