Complete PJRT copy futures via event callbacks instead of awaiter threads

[copybara-service]

Complete PJRT copy futures via event callbacks instead of awaiter threads

PjRtCopyFuture::OnReady parked a detached thread in PJRT_Event_Await per
in-flight copy; on the transfer receive path that is one short-lived
thread per layer per transfer, and sustained decode-side load crashed
schedulers with a segfault in the detach on the push-handler path.
Register a PJRT_Event_OnReady callback per event instead, joined by a
countdown that preserves the first error; completion callbacks now run
on PJRT completion threads rather than a dedicated awaiter.

Core dump before the CL:
https://github.com/sgl-project/sglang-torchtpu/actions/runs/31962643994

!!!!!!! Segfault encountered !!!!!!!
  File "<unknown>", line 0, in raise
  File "<unknown>", line 0, in pthread_detach
  File "<unknown>", line 0, in std::thread::detach()
  File "<unknown>", line 0, in tpu_raiden::KVCacheManagerWithTransfer::OnLayerReceived(unsigned long, unsigned long)
  File "<unknown>", line 0, in tpu_raiden::transport::BlockTransport::HandleIncomingPush(int, tpu_raiden::transport::lib::ChunkHeader const&)
  File "<unknown>", line 0, in tpu_raiden::transport::BlockTransport::HandleCustomRequest(int, tpu_raiden::transport::lib::ChunkHeader const&)
  File "<unknown>", line 0, in tpu_raiden::transport::lib::RawBufferTransport::ProcessPeerRequest(int)
  File "<unknown>", line 0, in tpu_raiden::transport::lib::RawBufferTransport::ConnectionWorker(int)
  File "<unknown>", line 0, in 0xffffffffffffffff