KeyVaultForLinux fails to run

Environment:
- Plain VM (no ARC or scale set)
- OS Ubuntu 20.04 LTS
- KeyVaultForLinux version 2.2.2158.57

Settings used:
```
{
"settings": {
"secretsManagementSettings": {
"pollingIntervalInS": "3600",
"linkOnRenewal": "false",
"certificateStoreLocation": "/var/cert",
"requireInitialSync": true,
"observedCertificates": [
"https://cert-url"
]
}
},
"authenticationSettings": {
"msiEndpoint": "http://169.254.169.254/metadata/identity",
"msiClientId": "managed identity object ID"
}
}
```

Error produced: "Attempt to get non-existent AuthConfig". I can't find anything on this error anywhere online, not even a code search of this repo has anything.

AADSSHLoginForLinux works fine, I can use `az ssh` to connect.

Any pointers to what might cause this?

Start by reproducing the failure with KeyVaultForLinux 2.2.2158.57 on Ubuntu 20.04 using the supplied secretsManagementSettings and authenticationSettings. Trace the extension's authentication configuration handling around "Attempt to get non-existent AuthConfig" and compare the working AADSSHLoginForLinux setup; done means identifying the cause and documenting a verified resolution.

Written by the indexing model from the issue text.