fetchSubagentHistory

Fetch and restore internal messages for reconstructed subagents from their subgraph checkpoints. Should be called after reconstructSubagents to restore the full subagent conversation after a page refresh.

Subagent messages are persisted in the LangGraph checkpointer under a subgraph-specific checkpoint_ns (e.g. tools:<uuid>). This method discovers the correct namespace by inspecting the main thread's intermediate history checkpoints, where each pending task's checkpoint.checkpoint_ns identifies the subgraph. Tasks are matched to tool calls by their Send index (task.path[1]), which corresponds to the order of tool calls in the AI message — no deepagent-specific metadata required.