refactor(state): flatten registry acquire flow; restore mirror cleanup assertion

Post-merge review follow-ups on #100201:

- acquire(): drop the never-iterating 'while True' and the redundant
  'existing is not generation' half of the race check — after the retire
  path, generation is always None on the fresh-open leg, so 'existing is
  not None' is the complete condition. Same behavior, flat flow.
- test_mirror: the conversion to the shared registry dropped the
  cleanup assertion entirely; restore it by patching
  hermes_state.release_or_close and asserting the handle is released
  exactly once after _append_to_sqlite.

tests/gateway/test_mirror.py

@@ -119,18 +119,21 @@ class TestMirrorToSession:
 
 
 class TestAppendToSqlite:
-    def test_connection_is_closed_after_use(self, tmp_path):
-        """Verify _append_to_sqlite releases the shared SessionDB handle."""
+    def test_connection_is_released_after_use(self, tmp_path):
+        """Verify _append_to_sqlite returns the shared SessionDB reference."""
         from gateway.mirror import _append_to_sqlite
         mock_db = MagicMock()
+        released = []
 
-        with patch("hermes_state.get_shared_session_db", return_value=mock_db):
+        with patch("hermes_state.get_shared_session_db", return_value=mock_db), \
+             patch(
+                 "hermes_state.release_or_close",
+                 side_effect=lambda db: released.append(db),
+             ):
             _append_to_sqlite("sess_1", {"role": "assistant", "content": "hello"})
 
         mock_db.append_message.assert_called_once()
-        # Shared instances are released (not closed) — the registry owns close().
-        # release_shared_session_db is a module-level function, so verify the
-        # mock was passed to it by checking that append_message was called
-        # (the real release_shared_session_db on a MagicMock is a no-op since
-        # the mock isn't in the registry).
+        assert released == [mock_db], (
+            "the shared handle must be released exactly once after use"
+        )