### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Configuring GCP IAM authentication (via `cache_params.gcp_service_account` or `REDIS_GCP_SERVICE_ACCOUNT` env) against a clustered Redis (e.g., Google Memorystore for Valkey, which is always cluster-mode) causes the proxy to crashloop at startup with:

``` redis.exceptions.RedisClusterException: Redis Cluster cannot be connected. Please provide at least one reachable node: Authentication required. ```

The same setup works correctly **without** GCP IAM when a static password is provided via `cache_params.password`.

### Root cause

In `litellm/_redis.py::_get_redis_client_logic`, the GCP IAM branch installs a custom connection callback via `redis_connect_func`:

```python if _gcp_service_account is not None: redis_kwargs["redis_connect_func"] = create_gcp_iam_redis_connect_func( service_account=_gcp_service_account, ssl_ca_certs=_gcp_ssl_ca_certs ) redis_kwargs["redis_connect_func"]._gcp_service_account = _gcp_service_account ```

`redis_connect_func` is honored by `redis.Redis` connections, but **`redis.RedisCluster`'s boo…