fix: preallocate slice in tests and add ResponseHandled guard in toolloop

Fix prealloc linter failure in load_image_test.go.

Prevent double-resolving media by checking ResponseHandled in toolloop.go.

pkg/tools/load_image_test.go

@@ -124,7 +124,7 @@ func TestLoadImage_SuccessPath(t *testing.T) {
 		0xAE, 0x42, 0x60, 0x82, // CRC
 	}
 
-	var pngData []byte
+	pngData := make([]byte, 0, len(pngSignature)+len(ihdr)+len(iend))
 	pngData = append(pngData, pngSignature...)
 	pngData = append(pngData, ihdr...)
 	pngData = append(pngData, iend...)

pkg/tools/toolloop.go

@@ -190,7 +190,7 @@ func RunToolLoop(
 				Content:    contentForLLM,
 				ToolCallID: r.tc.ID,
 			}
-			if len(r.result.Media) > 0 {
+			if len(r.result.Media) > 0 && !r.result.ResponseHandled {
 				toolMsg.Media = append(toolMsg.Media, r.result.Media...)
 			}
 			messages = append(messages, toolMsg)