Malformed username error not clear

[mcountryman]

Describe the bug
When using a username/password login with a username that does not end with @* the error provided is not clear as to what the issue is. I was able to peek at the code to identify the issue so this is partially a submission to document what I found and partially because I may or may not try to make a PR if I have time.

To Reproduce
Steps to reproduce the behavior:

1. Create a resource '....'
2. Run '....'
3. See error
4. Modify the resource '....'
5. Run '....'
6. See error

Please also provide a minimal Terraform configuration that reproduces the issue.

terraform {
  required_providers {
    proxmox = {
      source = "bpg/proxmox"
      version = "0.70.1"
    }
  }
}

provider "proxmox" {
  endpoint = var.proxmox-endpoint
  username = "root" # Missing `@pam`
}

# Any sort of resource definition to kick off the provider.  I didn't get far
# enough to actually validate below works.
resource "proxmox_virtual_environment_download_file" "nixos_lxc_25_05" {
  content_type = "vztmpl"
  datastore_id = "local"
  node_name    = "first-node"
  url          = "https://hydra.nixos.org/build/287411733/download/1/nixos-system-x86_64-linux.tar.xz"
}

and the output of terraform|tofu apply.

> tofu plan --var-file=.tfvars

Planning failed. OpenTofu encountered an error while generating this plan.

╷
│ Error: Unable to create Proxmox VE API credentials
│
│   with provider["registry.opentofu.org/bpg/proxmox"],
│   on providers.tf line 10, in provider "proxmox":
│   10: provider "proxmox" {
│
│ must provide either user credentials, an API token, or a ticket

Expected behavior
An error indicating that user cred validation failed rather than wasn't supplied.

Additional context
This is probably the issue here:

terraform-provider-proxmox/proxmox/api/credentials.go

Line 58 in 9bd0b57

usr, err := newUserCredentials(username, password, otp)

Looks like the validation logic and variable presence logic was merged leading to the need to throw away errors.

• Single or clustered Proxmox: Irrelevant
• Proxmox version: Irrelevant
• Provider version (ideally it should be the latest version): 0.70.1
• Terraform/OpenTofu version: Terraform v1.9.8/OpenTofu v1.8.7
• OS (where you run Terraform/OpenTofu from): darwin_arm64
• Debug logs (TF_LOG=DEBUG terraform apply):