### Check for existing issues

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

### What happened?

## Description Anthropic Messages passthrough does not honor `use_bearer_for_custom_base` when generating auth headers.

For custom Anthropic-compatible `/v1/messages` gateways such as Cloudflare AI Gateway, the upstream expects:

Authorization: Bearer <token>

But LiteLLM still sends:

x-api-key: <token>

## Repro Configure a model with:

model: anthropic/anthropic/claude-fable-5 api_base: https://api.cloudflare.com/client/v4/accounts/***/ai api_key: *** use_bearer_for_custom_base: true

Call `/v1/messages`.

## Expected LiteLLM sends `Authorization: Bearer <token>` to the custom API base.

## Actual LiteLLM sends `x-api-key: <token>`, causing upstream 401 Authentication error.

## Cause `AnthropicMessagesConfig.validate_anthropic_messages_environment()` calls:

AnthropicModelInfo.get_auth_header(api_key)

without passing `api_base` or `use_bearer_for_custom_base`.

## Fix Read `api_base` and `use_bearer_for_custom_base` from `litellm_params` and pass them into `get_auth_header()`.

### Steps to Reproduce

1. config.yaml

``` model_list: - …