### Check for existing issues

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

### What happened?

## Bug Description

When using the Anthropic batch API through LiteLLM, `CheckBatchCost` always records 0 tokens and $0 cost for completed batches, silently dropping all cost data.

## Root Cause

`AnthropicFilesConfig.transform_file_content_request` in `litellm/llms/anthropic/files/transformation.py` generates the wrong URL for batch result IDs:

```python # Current (broken) — always uses the Files API endpoint return f"{api_base.rstrip('/')}/v1/files/{file_id}/content", {}

### Steps to Reproduce

1. Send batch to anthropic model 2. Check the costs after the polling has completed 3. Costs remain 0

### Relevant log output

```shell

```

### What part of LiteLLM is this about?

Proxy

### What LiteLLM version are you on ?

v1.83.10

### Twitter / LinkedIn details

_No response_