Fix pixelmatch type error breaking CI builds (#2199)

The `diffBuf` variable was typed as `Uint8Array | null`, but pixelmatch's
third parameter expects `Uint8Array | undefined` (not null). Changed to
use `undefined` for the pixelmatch call and `?? null` for the return value
which expects `TypedArray | null`.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: amikofalvy

agents-manage-ui/vitest.config.ts

@@ -130,15 +130,15 @@ export default defineConfig({
                         ? (actual.data as Uint8Array)
                         : cropRgba(actual.data, aW, w, h);
 
-                    const diffBuf = createDiff ? new Uint8Array(w * h * 4) : null;
+                    const diffBuf = createDiff ? new Uint8Array(w * h * 4) : undefined;
                     const mismatched = pixelmatch(refData, actData, diffBuf, w, h, { threshold });
 
                     const total = w * h;
                     const ratio = mismatched / total;
 
                     return {
                       pass: ratio <= allowedMismatchedPixelRatio,
-                      diff: diffBuf,
+                      diff: diffBuf ?? null,
                       message:
                         ratio > allowedMismatchedPixelRatio
                           ? `${mismatched} pixels (${(ratio * 100).toFixed(2)}%) mismatched, allowed: ${(allowedMismatchedPixelRatio * 100).toFixed(2)}%`