### Check for existing issues

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

### What happened?

I cannot use S3 callbacks - they don't work in "test callback" nor in standard operation mode.

### Steps to Reproduce

1. Run LiteLLM proxy via Docker 2. Settings -> Logging & Alerts -> Callbacks -> Add callback 3. Select S3, fill in credentials (I'm using Session Token too) 4. Save 5. Click `Test callback` button

<img width="2543" height="516" alt="Image" src="https://github.com/user-attachments/assets/dd083e13-10c4-44e1-b6ac-5813039ed54d" />

For context: I added a custom webhook callback to see if it works at all - and indeed my webhook **does receive** the callback, so it's most def S3 integraiton that is broken.

### Relevant log output

```shell 10:46:07 - LiteLLM Proxy:ERROR: _health_endpoints.py:465 - litellm.proxy.proxy_server.health_services_endpoint(): Exception occured - 400: {'error': "Service must be in list. Service=s3 not in typing.Union[typing.Literal['slack_budget_alerts', 'langfuse', 'langfuse_otel', 'slack', 'openmeter', 'webhook', 'email', 'braintrust', 'datadog', 'datadog_llm_observability', 'generic_api', 'arize', 'sqs'…