[fix][ci] Format Tinker engine test

[bvolpato]

Status

Superseded by #2014, which merged the same Black formatting fix into main.

[gemini-code-assist]

Code Review

This pull request introduces minor formatting adjustments in tests/tinker/test_engine.py by adding blank lines before the forward_backward_payload function. There are no review comments, and I have no feedback to provide.

[bvolpato]

this is now blocking code-quality checks on #1998 and the newer drafts (#2005, #2006, #2007, and #2010). The patch is formatting-only, and its local pre-commit and focused test validation are green. Could we land it so those branches can rerun against formatted main?

[bvolpato]

Superseded by #2014, which merged the same Black formatting fix into main.