[Misc][PD] Disable bidirectional xfer mode for NixlPushConnector (#46473)

Signed-off-by: NickLucche <nicolo.lucchesi@mistral.ai>

vllm/distributed/kv_transfer/kv_connector/v1/nixl/push_scheduler.py

@@ -67,6 +67,10 @@ class NixlPushConnectorScheduler(NixlBaseConnectorScheduler):
         kv_cache_config: KVCacheConfig,
     ):
         super().__init__(vllm_config, engine_id, kv_cache_config)
+        if self.is_bidirectional_kv_xfer_enabled:
+            raise NotImplementedError(
+                "Bidirectional KV transfer is not supported for NIXL push connector."
+            )
 
         # D-side: registration data to pass to D workers via metadata on
         # the next ``build_connector_meta`` call.