## What happened

When routing an Anthropic 1M-context request through `bedrock_converse`, LiteLLM forwards the user-supplied `betas: ["context-1m-2025-08-07"]` array as a top-level body field. AWS Bedrock's Converse API rejects this with HTTP 400:

``` {"message":"The model returned the following errors: betas: Extra inputs are not permitted"} ```

Anthropic-native, Bedrock Invoke, and Vertex AI all accept top-level `betas` directly, so this is Converse-specific.

## Reproducer

```bash curl -s -X POST http://127.0.0.1:4000/v1/messages \ -H "Authorization: Bearer sk-1234" \ -H "anthropic-version: 2023-06-01" \ -H "anthropic-beta: context-1m-2025-08-07" \ -d '{ "model": "claude-opus-4-7-bedrock-converse", "max_tokens": 256, "messages": [{"role": "user", "content": "<~210k tokens of context here>"}], "betas": ["context-1m-2025-08-07"] }' ```

Returns: ```json {"error": {"message": "litellm.BadRequestError: BedrockException - {\"message\":\"The model returned the following errors: betas: Extra inputs are not permitted\"}", "code": "400"}} ```

The same request shape against `claude-opus-4-7-bedrock-invoke` succeeds (200), and against anthropic-native + verte…