[python] Add file_source assertion in row-tracking commit to fail fast on missing file_source (#7610)

if file_source is None, the condition `None == 0` silently evaluates to
False, skipping row ID assignment. This causes nextRowId not to
increment, leading to first_row_id conflicts. So this PR add file_source
check like java.

Author: littlecoder04

paimon-python/pypaimon/write/file_store_commit.py

@@ -697,6 +697,9 @@ def _assign_row_tracking_meta(self, first_row_id_start: int, commit_entries: Lis
         blob_start_by_field = {}
 
         for entry in commit_entries:
+            assert entry.file.file_source is not None, \
+                f"file_source must be present for row-tracking table, file={entry.file.file_name}"
+
             # Check if this is an append file that needs row ID assignment
             if (entry.kind == 0 and  # ADD kind
                     entry.file.file_source == 0 and  # APPEND file source