fix: Use HfApi.upload_file for more reliable qlog uploads

Changed from HfFileSystem (which had issues with append mode) to
HfApi.upload_file which is more reliable:
- Downloads existing content first
- Appends new entries
- Uploads combined content with commit message

Added more detailed logging to track upload progress.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/rag_chatbot/qlog/hf_writer.py

@@ -542,7 +542,7 @@ class HFDatasetWriter:
         try:
             # Lazy import huggingface_hub
             from huggingface_hub import (  # type: ignore[attr-defined]
-                HfFileSystem as _HfFileSystem,
+                HfApi as _HfApi,
             )
 
             # Build JSONL content from entries
@@ -553,22 +553,52 @@ class HFDatasetWriter:
 
             jsonl_content = "\n".join(jsonl_lines) + "\n"
 
-            # Use HfFileSystem for append operation
-            # This is more efficient than downloading, appending, and re-uploading
-            fs = _HfFileSystem(token=self._hf_token)
-            file_path = f"datasets/{self._repo_id}/{QLOG_FILENAME}"
+            logger.info(
+                "Uploading %d log entries to repo '%s'...",
+                len(entries),
+                self._repo_id,
+            )
+
+            # Use HfApi.upload_file for more reliable uploads
+            # This creates the file if it doesn't exist and appends if it does
+            api = _HfApi(token=self._hf_token)
 
-            # Append to the file (creates if doesn't exist)
-            # Note: HfFileSystem.open() with mode 'a' appends to existing content
+            # First, try to download existing content to append
+            existing_content = ""
             try:
-                with fs.open(file_path, mode="a") as f:
-                    f.write(jsonl_content)
-            except FileNotFoundError:
-                # File doesn't exist, create it with write mode
-                with fs.open(file_path, mode="w") as f:
-                    f.write(jsonl_content)
-
-            logger.debug("Uploaded %d entries to %s", len(entries), file_path)
+                from huggingface_hub import hf_hub_download
+
+                local_path = hf_hub_download(
+                    repo_id=self._repo_id,
+                    filename=QLOG_FILENAME,
+                    repo_type="dataset",
+                    token=self._hf_token,
+                )
+                with open(local_path, "r", encoding="utf-8") as f:
+                    existing_content = f.read()
+                logger.debug("Loaded existing qlog with %d bytes", len(existing_content))
+            except Exception:
+                # File doesn't exist yet, that's OK
+                logger.debug("No existing qlog file, will create new one")
+
+            # Combine existing content with new entries
+            combined_content = existing_content + jsonl_content
+
+            # Upload the combined content
+            api.upload_file(
+                path_or_fileobj=combined_content.encode("utf-8"),
+                path_in_repo=QLOG_FILENAME,
+                repo_id=self._repo_id,
+                repo_type="dataset",
+                commit_message=f"Add {len(entries)} query log entries",
+            )
+
+            logger.info(
+                "Successfully uploaded %d entries to %s/%s",
+                len(entries),
+                self._repo_id,
+                QLOG_FILENAME,
+            )
 
         except ImportError:
             logger.exception("huggingface_hub not installed, cannot upload logs")