#7786 Fix Sonner toast button style prop names

Closes #7785

Summary

• Serialize toast action and cancel button styles using Sonner's singular option names.
• Preserve the existing Python API field names for compatibility.
• Add a regression test for the JSON sent to the frontend and a package bugfix news fragment.

Validation

• pytest tests/units/reflex_components_sonner/test_toast.py -q (1 passed).
• Ruff check/format, Pyright, and codespell passed for the changed files.
• python scripts/make_pyi.py --check passed.
• Playwright was not run; the regression is directly covered at the serialized JSON boundary.