test(media): match the overflow placeholder by const, not substring (gadfly #8)

ragnaros/qwen3.6-27b noted TestNormalizeOverCount matched 'omitted' by substring;
the test is in-package, so assert == imageOverflowPlaceholder instead — robust to
wording changes. No behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

media/media_test.go

@@ -173,7 +173,7 @@ func TestNormalizeOverCount(t *testing.T) {
 			case llm.ImagePart:
 				imgs = append(imgs, v)
 			case llm.TextPart:
-				if strings.Contains(v.Text, "omitted") {
+				if v.Text == imageOverflowPlaceholder {
 					placeholders++
 				}
 			}