Chain fetcher errors in composer lookup

Keep track of the last exception raised by candidate fetchers and attach it as the cause when no fetcher succeeds. This preserves the underlying failure context (for example remote HTTP/auth errors) while still raising the existing ValueError with supported fetchers.

Author: JR-1991

pyenzyme/composer.py

@@ -197,17 +197,21 @@ def _fetch_with_fetchers(
     Raises:
         ValueError: If no fetcher can handle the given entity ID
     """
+    last_error: Optional[Exception] = None
     for fetcher in fetchers:
         try:
             return fetcher(entity_id)
-        except Exception:
+        except Exception as e:
+            last_error = e
             continue
 
     fetcher_names = ", ".join(f.__name__ for f in fetchers)
+    # Chain the last underlying error so callers can inspect the real cause
+    # (e.g. an HTTP 403 from a remote database blocked in CI).
     raise ValueError(
         f"No {entity_type} fetcher found for {entity_id}. "
         f"Supported fetchers: {fetcher_names}"
-    )
+    ) from last_error
 
 
 def _remove_duplicates(objects: List[Any]) -> List[Any]: