fix(sdk): defer stream join until lazy thread create commits (#2677)

Hydrating an externally-minted thread id that 404s still opened
`/stream/events` before `POST /commands` created the row. On
langgraph_api's in-mem runtime that join is accepted but dead, so the
first run delivered nothing until idle reconnect. Treat missing threads
like client-minted ones and start the root pump only after dispatch
succeeds.