you are resetting the alpha of Particles to 1 when you detect you are wallsliding, but this won't work because you overwrite it in this case anyway with your alpha variable, which you don't reset.
What you should do, is reset the variable alpha to 1 any time you detect, that the character is not wallriding. As your code is structured you don't need to reset the alpha of your particles to 1, because you overwrite it with your alpha variable anyway.
Without testing it, I think it should look like this:
if is_on_wall() and not is_on_floor():
if input.is_action_pressed("ui_right") or Input.is_action_pressed("ui_left"):
is_wall_sliding = true
else:
is_wall_sliding = false
else:
is_wall_sliding = false
if is_wall_sliding == true:
velocity.y = wall_slide_gravity
sparks_fade()
else:
alpha = 1