test(state): reset the single-flight _opening map in the registry fixture

The _clean_registry fixture clears _generations and _retired between
tests; the new _opening map needs the same reset so a test that aborts
mid-construction cannot leave a stale opening event that stalls the
next test's cold acquire.

tests/hermes_state/test_shared_session_db_registry.py

@@ -35,10 +35,12 @@ def _clean_registry():
     registry.close_all()
     registry._generations.clear()
     registry._retired.clear()
+    registry._opening.clear()
     yield
     registry.close_all()
     registry._generations.clear()
     registry._retired.clear()
+    registry._opening.clear()
 
 
 def _replace_file_preserving_schema(src: Path, dst: Path) -> None: