ServiceTier when using Vertex AI

When using Vertex AI, if I try to set the ServiceTier (e.g: FLEX), I receive the following error:

```java
{
"error": {
"code": 400,
"message": "Invalid value at 'service_tier' (type.googleapis.com/google.cloud.aiplatform.v1beta1.ServiceTier), \"flex\"",
"status": "INVALID_ARGUMENT",
"details": [
{
"@type": "type.googleapis.com/google.rpc.BadRequest",
"fieldViolations": [
{
"field": "service_tier",
"description": "Invalid value at 'service_tier' (type.googleapis.com/google.cloud.aiplatform.v1beta1.ServiceTier), \"flex\""
}
]
}
]
}
}
```

This problem seems very similar to: https://github.com/googleapis/js-genai/issues/1468 in particular see https://github.com/googleapis/js-genai/issues/1468#issuecomment-4229949570