fix: handle empty suffix in removesuffix - #190

Summary

• guard suffix removal against the empty-suffix slicing edge case
• compare both compatibility helpers with the Python standard library
• cover matching, non-matching, empty-affix, and empty-input cases

Why

string[:-len(suffix)] becomes string[:0] for an empty suffix, returning an empty string instead of preserving the input as str.removesuffix("") does.

Fixes #182

Testing

• python -m pytest tests/test_utils/test_utils.py -q (8 passed)
• python -m mypy src/browserbase/_utils/_utils.py tests/test_utils/test_utils.py
• python -m pyright src/browserbase/_utils/_utils.py tests/test_utils/test_utils.py
• python -m ruff check src/browserbase/_utils/_utils.py tests/test_utils/test_utils.py
• python -m ruff format --check src/browserbase/_utils/_utils.py tests/test_utils/test_utils.py
• full suite: 1225 passed, 8 skipped; 4 pre-existing Windows/environment failures fixed separately by test: make suite deterministic on Windows #189
• full suite excluding only the four test: make suite deterministic on Windows #189 cases: 1225 passed, 8 skipped