生成图片的 gridX/gridY 解析不再把超大整数截断成合法网格

- generate_image_grid_axis 原来用 `value as u32`，u64 直接截断：4294967297（2^32+1）会变成 1，随后 1..=32 的区间校验把这个巨大输入当成合法网格静默接受
- 改成 u32::try_from，超出 u32 的值直接落到 GridAxisNotInteger 拒绝分支，不再依赖截断后的值
- 补一条回归测试：gridX=4294967297 必须被拒绝，gridX=4/gridY=4 仍通过

验证：cargo test --bin genarrative-ai-game-creator-shell -- agent::direct_tool_bridge::tests::generate_image_grid_axis_rejects_values_beyond_u32 通过

apps/ai-game-creator-shell/src-tauri/src/agent/direct_tool_bridge.rs

@@ -2850,7 +2850,8 @@ fn generate_image_grid_axis(
     };
     value
         .as_u64()
-        .map(|value| Some(value as u32))
+        .and_then(|value| u32::try_from(value).ok())
+        .map(Some)
         .ok_or(GenerateImageError::GridAxisNotInteger { axis })
 }
 
@@ -4772,6 +4773,30 @@ mod tests {
         .is_err());
     }
 
+    #[test]
+    fn generate_image_grid_axis_rejects_values_beyond_u32() {
+        let error = match generate_image_input(&json!({
+            "prompt": "像素风图标集",
+            "kind": "icon-spritesheet",
+            "sliceMode": "grid",
+            "gridX": 4_294_967_297_u64,
+            "gridY": 1
+        })) {
+            Ok(_) => panic!("gridX 超过 u32 时不得截断成合法网格"),
+            Err(error) => error,
+        }
+        .to_user_msg();
+        assert!(error.contains("gridX"), "{error}");
+        assert!(generate_image_input(&json!({
+            "prompt": "像素风图标集",
+            "kind": "icon-spritesheet",
+            "sliceMode": "grid",
+            "gridX": 4,
+            "gridY": 4
+        }))
+        .is_ok());
+    }
+
     #[test]
     fn large_mcp_images_are_reduced_to_bounded_jpeg_previews() {
         let image = image::RgbaImage::from_fn(1600, 1200, |x, y| {