Add API workflow JSON demonstrating ImageBlend channel-count fix

Adds a self-contained API workflow at
tests/inference/graphs/image_blend_channel_mismatch.json that exercises
the previously-broken RGB + RGBA blend case end-to-end:

  EmptyImage (RGB, red)        -> image1
  EmptyImage (RGB, blue)
    + SolidMask (0.5)          -> JoinImageWithAlpha (RGBA) -> image2
                                 -> ImageBlend (3ch + 4ch)
                                 -> SaveImage

The workflow uses no model checkpoints and runs in seconds on CPU. With
the fix it produces a 4-channel RGBA output (alpha preserved); the
previous behavior would have silently dropped the alpha channel via
node_helpers.image_alpha_fix.

Verified end-to-end against a local ComfyUI instance: workflow executes
successfully, output PNG is RGBA, center pixel is (127, 0, 127, 191) -
red and blue blended at 0.5 with alpha 0.5*1.5=0.75 (191/255).

tests/inference/graphs/image_blend_channel_mismatch.json

@@ -0,0 +1,69 @@
+{
+    "1": {
+        "inputs": {
+            "width": 256,
+            "height": 256,
+            "batch_size": 1,
+            "color": 16711680
+        },
+        "class_type": "EmptyImage",
+        "_meta": {
+            "title": "RGB image (3 channels, red)"
+        }
+    },
+    "2": {
+        "inputs": {
+            "width": 256,
+            "height": 256,
+            "batch_size": 1,
+            "color": 255
+        },
+        "class_type": "EmptyImage",
+        "_meta": {
+            "title": "Base image for RGBA (blue)"
+        }
+    },
+    "3": {
+        "inputs": {
+            "value": 0.5,
+            "width": 256,
+            "height": 256
+        },
+        "class_type": "SolidMask",
+        "_meta": {
+            "title": "Alpha mask (0.5)"
+        }
+    },
+    "4": {
+        "inputs": {
+            "image": ["2", 0],
+            "alpha": ["3", 0]
+        },
+        "class_type": "JoinImageWithAlpha",
+        "_meta": {
+            "title": "RGBA image (4 channels)"
+        }
+    },
+    "5": {
+        "inputs": {
+            "image1": ["1", 0],
+            "image2": ["4", 0],
+            "blend_factor": 0.5,
+            "blend_mode": "normal"
+        },
+        "class_type": "ImageBlend",
+        "_meta": {
+            "title": "Blend RGB (3ch) + RGBA (4ch) - exercises CORE-103 fix"
+        }
+    },
+    "6": {
+        "inputs": {
+            "filename_prefix": "image_blend_channel_test",
+            "images": ["5", 0]
+        },
+        "class_type": "SaveImage",
+        "_meta": {
+            "title": "Save blended output (will be RGBA)"
+        }
+    }
+}