fix(sprite): verify the sprites exist before slice_sheet reports them

The previous commit closes the one reproducible way Unity accepts a grid and
emits nothing. It is not the only way: Unity reports an import that generates no
sprite through the console, not through an exception or a return value, so any
future cause lands as the same silent success. Counting the sprites actually on
the asset answers that without having to enumerate the causes.

No dedicated test: the only trigger that reproduces on demand is the NPOT path,
and the commit before this one fixes that path rather than leaving it available
to assert on. Every existing slice test asserts the sprite count through the
Slice fixture helper, so the invariant this guard enforces is already the one
they measure.

Author: Scriptwonder

MCPForUnity/Editor/Tools/Sprite2D/SpriteImportSetup.cs

@@ -312,6 +312,18 @@ private static object SliceConverted(JObject @params, SpriteDiagnosticBuilder di
             EditorUtility.SetDirty(importer);
             importer.SaveAndReimport();
 
+            // Unity can accept every SpriteMetaData entry and still emit no sprite for it, and
+            // it says so in the console rather than throwing. NPOT scaling was one such path
+            // and is closed above; an import that fails for any other reason would report the
+            // same success over an empty asset. Counting what is actually on the asset is the
+            // only answer that does not depend on knowing the causes in advance.
+            int generated = AssetDatabase.LoadAllAssetsAtPath(path).OfType<Sprite>().Count();
+            if (generated != totalFrames)
+                return diagnostics.Fail("SLICE_NOT_GENERATED",
+                    $"Unity accepted a {cols}x{rows} grid but generated {generated} of {totalFrames} sprites for '{path}'.",
+                    "Check the Unity console for the import error",
+                    "Confirm the texture's import settings allow sprite generation");
+
             return new
             {
                 success      = true,