Expose draft tree traversal on GrammarMatcher (#613)

## Summary
- Move draft tree traversal into the public GrammarMatcher API for C++
and Python callers.
- Keep the Python testing wrapper as a backward-compatible shim while
removing the C++ testing free function.
- Update speculative decoding tests to cover the member API and
compatibility wrapper.

## Validation
- cmake --build build -j2
- python -m pytest tests/python/test_speculative_decoding.py
- python -m ruff check python/xgrammar/matcher.py
python/xgrammar/testing.py tests/python/test_speculative_decoding.py
- git diff --check