Optimize join function for base URL with trailing slash case
Reduce string operations when both parts have separator at boundary.
Instead of slicing part1, removing separator, and re-adding it,
simply concatenate part1 with part2 (minus separator).
Performance improvement: ~19% faster for URL joining with trailing slash
Co-authored-by: hckhanh <6380436+hckhanh@users.noreply.github.com>