fix(clifmt): ensure diff background color fills entire line width
Chroma syntax highlighter may inject its own background colors (e.g.,
monokai style on # comments). The previous code only replaced \\x1b[0m
in the highlighted text, but if Chroma set a custom background that
wasn't followed by \\x1b[0m, the \\x1b[K sequence at line end would use
the wrong background.
Fix: re-apply the diff background color immediately before \\x1b[K so
the erase-to-EOL always uses the correct diff background.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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