Eu multi-region not compatible with init

#### Environment details

- OS type and version: Mac and Linux
- Python version: `3.13.2`
- `google-cloud-aiplatform` version: `1.153.1`

#### Steps to reproduce

1. Initialize vertex client through vertexai.init or aiplatform.init with location = "eu"
2. It fails because its not considered a valid location

#### Code example

```python
from google.cloud import aiplatform
aiplatform.init(location="eu")
```

#### Stack trace
```
ValueError: Unsupported region for Vertex AI, select from frozenset({'asia-northeast3', 'us-west1', 'europe-west6', 'europe-west4', 'europe-central2', 'europe-west12', 'europe-west9', 'us-east1', 'me-central2', 'europe-west8', 'europe-north2', 'us-east5', 'australia-southeast2', 'asia-northeast1', 'asia-southeast2', 'southamerica-east1', 'northamerica-northeast1', 'africa-south1', 'europe-west1', 'us-east7', 'us-east4', 'asia-east2', 'us-south1', 'southamerica-west1', 'europe-west2', 'europe-west3', 'northamerica-northeast2', 'us-west4', 'us-central1', 'asia-south2', 'me-central1', 'us-west3', 'asia-northeast2', 'global', 'europe-north1', 'asia-south1', 'australia-southeast1', 'asia-east1', 'europe-southwest1', 'asia-southeast1', 'me-west1', 'us-west8', 'us-west2'})
```

Reproduce the failure with aiplatform.init(location="eu") and vertexai.init using the versions and environments listed. Trace the location validation that raises the unsupported-region ValueError, then update the relevant behavior so the multi-region value is accepted and verify that both initialization paths succeed.

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