Don't invert the alpha channel in the Invert Image node

ImageInvert applied 1.0 - image across the whole tensor, so a 4 channel
RGBA image came back with its transparency flipped: opaque pixels became
transparent and vice versa. Alpha stores coverage, not color, so leave it
alone and only invert RGB. Matches GIMP, ImageMagick and Blender, and the
existing srgb_to_linear/hlg_to_linear handling in nodes_images.py.

Reported in #5610.

nodes.py

@@ -1947,6 +1947,8 @@ class ImageInvert:
 
     def invert(self, image):
         s = 1.0 - image
+        if image.shape[-1] == 4:  # alpha stores transparency, not color
+            s[..., 3] = image[..., 3]
         return (s,)
 
 class ImageBatch:

tests-unit/comfy_test/image_invert_test.py

@@ -0,0 +1,46 @@
+import torch
+
+from comfy.cli_args import args as cli_args
+
+if not torch.cuda.is_available():
+    cli_args.cpu = True
+
+import nodes  # noqa: E402
+
+
+def test_invert_rgb_inverts_every_channel():
+    image = torch.rand(1, 4, 4, 3)
+
+    s, = nodes.ImageInvert().invert(image)
+
+    assert s.shape == image.shape
+    assert torch.allclose(s, 1.0 - image)
+
+
+def test_invert_rgba_leaves_alpha_untouched():
+    image = torch.rand(2, 4, 4, 4)
+
+    s, = nodes.ImageInvert().invert(image)
+
+    assert s.shape == image.shape
+    assert torch.allclose(s[..., :3], 1.0 - image[..., :3])
+    assert torch.equal(s[..., 3], image[..., 3])
+
+
+def test_invert_rgba_keeps_transparent_pixels_transparent():
+    image = torch.zeros(1, 1, 2, 4)
+    image[..., 3] = torch.tensor([1.0, 0.0])
+
+    s, = nodes.ImageInvert().invert(image)
+
+    assert s[0, 0, 0, 3] == 1.0
+    assert s[0, 0, 1, 3] == 0.0
+
+
+def test_invert_does_not_mutate_input():
+    image = torch.rand(1, 4, 4, 4)
+    original = image.clone()
+
+    nodes.ImageInvert().invert(image)
+
+    assert torch.equal(image, original)