fix(vm_provision_mixin): upload Cloud-Init snippet as a real multipart file

proxmoxer only builds a multipart request for io.IOBase values passed
as kwargs; a plain filename string (plus a nonexistent "data" field,
as the old code sent) goes out as an ordinary form-urlencoded POST
instead. Real Proxmox's /storage/{s}/upload endpoint expects an actual
file upload for "filename" and responds to anything else by closing
the connection with no HTTP response at all.

Found live: the VM shell, disk import, and node-scoped storage
selection all succeeded, then create_vm_from_cloud_init failed with
requests.exceptions.ConnectionError / RemoteDisconnected right at the
snippet upload step.

napalm_proxmox/vm_provision_mixin.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import io
 import logging
 import time
 import yaml
@@ -337,11 +338,18 @@ class ProxmoxVMProvisionMixin:
             filename = f"{vmid}-user-data.yaml"
             _logger.debug(f"Uploading Cloud-Init snippet {filename} to {snippet_storage}")
 
-            # Upload to snippet storage
+            # Upload to snippet storage. Proxmox's upload endpoint expects the
+            # "filename" parameter to BE the file (multipart), not a name
+            # string with separate content — proxmoxer only builds a
+            # multipart request when the value is an io.IOBase instance,
+            # otherwise it silently sends everything as a plain
+            # form-urlencoded POST, which real Proxmox rejects by dropping
+            # the connection (RemoteDisconnected, no HTTP response at all).
+            file_obj = io.BytesIO(user_data_yaml.encode("utf-8"))
+            file_obj.name = filename
             self._node_api().storage(snippet_storage).upload.post(
                 content="snippets",
-                filename=filename,
-                data=user_data_yaml,
+                filename=file_obj,
             )
 
             # Step 7: Configure Cloud-Init references and SSH keys