Function validate_per_param_attr_conflicts 

pub(crate) fn validate_per_param_attr_conflicts(
    attr: &PerParamMiniextendrAttr,
    param_name: &str,
    is_dots: bool,
    ty: Option<&Type>,
    span: Span,
) -> Result<()>

Validate per-parameter attribute conflicts.

Returns Err if:

• coerce + match_arg on the same parameter
• coerce + choices(...) on the same parameter
• choices(...) + explicit default on the same parameter
• default on a &Dots parameter