clarify start-label validation message

Author: Copilot

poseinterface/clips.py

@@ -25,7 +25,7 @@ def _extract_start_labels(clip_labels: dict) -> dict:
     start_images = [img for img in clip_labels["images"] if img["id"] == 0]
     if len(start_images) != 1:
         raise ValueError(
-            "Clip labels must contain exactly one image with id 0"
+            "Clip labels must contain exactly one first-frame image with id 0"
         )
 
     return {
@@ -363,6 +363,7 @@ def extract_start_frames_label_s3(
             raise FileNotFoundError(
                 f"File not found on S3: {s3_cliplabels_uri}"
             ) from e
+        # Preserve other S3 client errors, but log them for easier diagnosis.
         logging.error(
             "Failed to extract start labels from %s (S3 error: %s)",
             s3_cliplabels_uri,

tests/test_unit/test_clips.py

@@ -180,7 +180,10 @@ def test_extract_start_labels_requires_first_frame(video_labels):
         "annotations": [{"image_id": 1, "id": 1}],
     }
 
-    with pytest.raises(ValueError, match="exactly one image with id 0"):
+    with pytest.raises(
+        ValueError,
+        match="exactly one first-frame image with id 0",
+    ):
         _extract_start_labels(clip_labels)