Function scan_model_switch 

pub fn scan_model_switch(
    diff: &str,
    harness: &str,
    model_config: &ModelConfig,
) -> ModelSwitchScan

Scan a unified diff for an inline /model <x> command in user-added lines.

Behavior:

• Only matches + lines (user additions), excluding +++ headers.
• Skips lines inside fenced code blocks (``` or ~~~).
• Skips blockquote lines (+>).
• Pattern: line content matches /model <arg> (whitespace allowed).
• On match, the line is removed from the returned diff so it does not propagate to classification or response generation.
• Only the first match is captured; subsequent /model lines are still stripped.

The arg is parsed via parse_model_arg, which accepts both tier names (low|med|high) and concrete model names (opus|sonnet|...).