fix: add explanatory comment for empty except clause

Co-Authored-By: sophie.cui@airbyte.io <sophie.cui@airbyte.io>

Author: devin-ai-integration[bot]

airbyte_cdk/sources/declarative/expanders/record_expander.py

@@ -35,7 +35,7 @@ def copy_field(
             value = dpath.get(dict(parent_record), source_path)
             child_record[self.target_field] = value
         except KeyError:
-            pass
+            pass  # Missing source fields in parent record are expected and intentionally skipped
 
 
 @dataclass