fix(proxmox_lxc_provision): use literal module-name keys in module_defaults

The previous commit (d5cf6f6) used 'module_defaults: "{{ _proxmox_module_defaults }}"'
to template the entire defaults dict. Ansible rejects this because
module_defaults keys must be static action/module/group names at parse
time — only values may be templated.

Expose _proxmox_api_args as a flat dict of API parameters in
defaults/main.yml, and in each task file's module_defaults block use
the literal community.proxmox.proxmox and community.proxmox.proxmox_vm_info
keys with templated values pointing at the var.

roles/proxmox_lxc_provision/tasks/check-exists.yml

@@ -1,6 +1,8 @@
 ---
 - name: Check if LXC exists
-  module_defaults: "{{ _proxmox_module_defaults }}"
+  module_defaults:
+    community.proxmox.proxmox: "{{ _proxmox_api_args }}"
+    community.proxmox.proxmox_vm_info: "{{ _proxmox_api_args }}"
   block:
     - name: Query Proxmox for existing LXCs
       community.proxmox.proxmox_vm_info: