DSPy ReActV2 gets async execution and stops returning half-finished Predictions.

PRs #10355 and #10356 add await agent.acall(...) covering async prediction, async tools and forced final submission, with async MCP tools running through the agent's structured tool-call history and call IDs preserved across turns. Tools execute sequentially, so this adds no parallel execution and doesn't offload blocking sync tools. ReActV2 now raises ValueError on missing or invalid final submission instead of returning an incomplete Prediction, and rejects signatures with output fields named history or termination_reason.