Warn when VAE.encode drops input channels

VAE.encode silently trims anything past output_channels, so an RGBA image
loses its alpha with no log line. Warn once per VAE and point at
SplitImageWithAlpha. No behavior change.

tests-unit/comfy_test/vae_channel_trim_test.py

@@ -0,0 +1,42 @@
+import logging
+
+import torch
+
+from comfy.cli_args import args
+
+if not torch.cuda.is_available():
+    args.cpu = True
+
+from comfy.sd import VAE  # noqa: E402
+
+
+def make_vae():
+    vae = VAE(sd={})
+    vae.crop_input = False
+    return vae
+
+
+def test_extra_channels_are_trimmed_and_warned_once(caplog):
+    vae = make_vae()
+    pixels = torch.zeros(1, 16, 16, 4)
+
+    with caplog.at_level(logging.WARNING):
+        first = vae.vae_encode_crop_pixels(pixels)
+        second = vae.vae_encode_crop_pixels(pixels)
+
+    assert first.shape == (1, 16, 16, 3)
+    assert second.shape == (1, 16, 16, 3)
+
+    trim_warnings = [r for r in caplog.records if "VAE encode" in r.getMessage()]
+    assert len(trim_warnings) == 1
+    assert "SplitImageWithAlpha" in trim_warnings[0].getMessage()
+
+
+def test_matching_channels_do_not_warn(caplog):
+    vae = make_vae()
+
+    with caplog.at_level(logging.WARNING):
+        out = vae.vae_encode_crop_pixels(torch.zeros(1, 16, 16, 3))
+
+    assert out.shape == (1, 16, 16, 3)
+    assert not [r for r in caplog.records if "VAE encode" in r.getMessage()]