Fix branching chat edit/regenerate using wrong checkpoint in useStream (Vue)

[Copilot]

Edit and regenerate in the branching chat pattern were appending messages to the end of the conversation instead of branching from the correct checkpoint. The regenerate function was particularly broken: it traversed back to find the preceding human message and used that message's parent_checkpoint, going one step too far back in history.

Changes

• handleRegenerate logic fix: Remove the backward human-message search. Use the AI message's own parent_checkpoint directly — this is the state after the user input was committed but before the AI responded, which is the correct branching point for regeneration.

  // Before: found preceding human message, used its parent_checkpoint (one step too far back)
  // After: use the AI message's parent_checkpoint directly
  const msg = streamedMessages.value.find((m) => m.id === messageId);
  const checkpoint = getParentCheckpoint(msg); // state after user msg, before AI response
  stream.submit(undefined, { checkpoint });

• Type corrections: Replaced the nonexistent UIMessage import from 'ai' with BaseMessage from @langchain/core/messages (the actual runtime type). Updated all function signatures accordingly. Changed the HumanMessage/AIMessage import source from 'langchain' to '@langchain/core/messages'.

• Minor fixes: Explicit type on the .map() callback in getMessageText; msg.id ?? null to satisfy Ref<string | null> (since BaseMessage.id is string | undefined).

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