ensure_message_ids

Coerce message-like write values to typed BaseMessages with stable IDs.

Called in put_writes() before DeltaChannel writes are submitted to the checkpointer. Without this the checkpoint may store raw dicts or id=None BaseMessages; every get_state() replay then produces a different UUID and the same message appears with a different ID in each LangSmith trace.

Handles three input shapes:

• BaseMessage objects: assign a UUID if id is None.
• Dicts with a known "role" (OpenAI-style) or "type" (LangChain format) at the root level: stamp "id" into the dict in-place. The reducer's convert_to_messages call will forward the id to the resulting BaseMessage.
• Lists of the above: apply the same logic to each element, replacing dict items with coerced BaseMessages so the shared list reference seen by checkpoint_pending_writes and the background thread both get typed messages.

Mutating synchronously here (before the background thread is submitted) is safe: the serialised bytes always reflect the post-coercion state.