Fix ruff config: remove invalid line-length from [tool.ruff.format]

line-length is only valid under [tool.ruff], not [tool.ruff.format].
Having it in [tool.ruff.format] caused ruff to fail parsing pyproject.toml
and fall back to default settings (line-length=88), which then flagged
core.py as needing reformat.

Author: neluca

pyproject.toml

@@ -50,7 +50,6 @@ target-version = "py39"
 select = ["E", "F", "W", "I", "N", "UP", "B", "C4"]
 
 [tool.ruff.format]
-line-length = 120
 quote-style = "double"
 indent-style = "space"