"Error loading certificate" when referencing SSH private key

#### Overview of the Issue

`packer validate` returns error `ssh_private_key_file is invalid: Error loading certificate` when using field `ssh_private_key_file` in source qemu block.

The key file exists. The keypair has been generated from command `ssh-keygen -f rsa -t rsa` with no passphrase.
Also, `ed25519` keys have been tested, with the same packer results.

The private and public key pairs have been tested with commands:
```shell
PRIVKEY=rsa
TESTKEY=rsa.pub
diff <( ssh-keygen -y -e -f "$PRIVKEY" ) <( ssh-keygen -y -e -f "$TESTKEY" )
```
(Source: https://serverfault.com/questions/426394/how-to-check-if-an-rsa-public-private-key-pair-match)

#### Reproduction Steps

- Download attached archive, uncompress, cd in folder.
- Change èath values for `ssh_certificate_file` and `ssh_private_key_file` to reference the ssh keypair file paths in the archive.
- Change fields values `iso_url` and `iso_checksum`.
- Run `packer validate .`

### Packer Plugin SDK version

- packer version: v1.11.2
- packer qemu plugin: github.com/hashicorp/qemu 1.1.0

### Operating system and Environment details

Ubuntu Linux 24.04. Packer installed manually via binary download.

### Log Fragments and crash.log files

Running packer with `PACKER_LOG=1` does not add useful information about the key error.

In this repository (https://github.com/hashicorp/packer-plugin-sdk) I found that the string `Error loading certificate` is only present in file [communicator/ssh/ssh.go](https://github.com/hashicorp/packer-plugin-sdk/blob/main/communicator/ssh/ssh.go#L81) and is returned after the function [ssh.ParseAuthorizedKey](https://github.com/hashicorp/packer-plugin-sdk/blob/main/communicator/ssh/ssh.go#L73C22-L73C44), which does not seem to handle private keys but [authorized_keys files](https://pkg.go.dev/golang.org/x/crypto/ssh#ParseAuthorizedKey).

**Attachment:** [packer.zip](https://github.com/user-attachments/files/18390020/packer.zip)

Thank you for your support.