If the line is fully within a polygon geometry, the @within variable will return the the containing polygon and use its boundary as the placement, otherwise the original line geometry is used.

with_variable('within', overlay_within('poly', @geometry)[0],
    case 
        when @within is not NULL
        then @within
        else @geometry
    end
)

Using this expression, if the line is fully within a polygon geometry, the @within variable will return the the containing polygon and use its boundary as the placement. Otherwise, the original line geometry is used.

Change 'poly' for your polygon layer name.

with_variable('within', overlay_within('poly', @geometry)[0],
    case 
        when @within is not NULL
        then @within
        else @geometry
    end
)