### Check for existing issues

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

### What happened?

`/eu.assemblyai/*` requests on the built-in AssemblyAI passthrough route resolve to the default US AssemblyAI base URL instead of the EU AssemblyAI base URL.

In `assemblyai_proxy_route()` (`litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py`), region detection currently does:

```python assembly_region = AssemblyAIPassthroughLoggingHandler._get_assembly_region_from_url( url=str(request.url) ) ```

and `_get_assembly_region_from_url()` (`litellm/proxy/pass_through_endpoints/llm_provider_handlers/assembly_passthrough_logging_handler.py`) only returns `"eu"` when:

```python urlparse(url).hostname == "eu.assemblyai.com" ```

For a normal LiteLLM proxy deployment, a request like:

`https://proxy.company.com/eu.assemblyai/v2/transcript`

has hostname `proxy.company.com`, not `eu.assemblyai.com`, so the helper returns `None`. That makes the route fall back to the default region:

- `_get_assembly_base_url_from_region(None)` chooses `https://api.assemblyai.com` - `passthrough_endpoint_router.get_credentials("assemblyai", None)` use…