### Check for existing issues

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

### What happened?

Requests with `parallel_tool_calls: true` and `tools` to a Bedrock Claude 4.5 model fail with: ``` BedrockException - {"message":"The model returned the following errors: tool_choice.type: Field required"} ```

The error references snake_case `tool_choice.type` rather than Bedrock's `toolChoice`, which means it's coming from `additionalModelRequestFields` — passed through to the Anthropic model directly.

In `converse_transformation.py`, `map_openai_params()` creates a `_parallel_tool_use_config`:

```python optional_params["_parallel_tool_use_config"] = { "tool_choice": { "disable_parallel_tool_use": disable_parallel } } ```

For models matching `is_claude_4_5_on_bedrock()`, this gets merged into `additionalModelRequestFields`. The Anthropic model then rejects it because `tool_choice` is missing the required `"type"` field.

This doesn't affect older models (3.5/3.7) since those go through Bedrock's native `toolChoice` field instead.

Fix — add `"type": "auto"` to the config:

```diff optional_params["_parallel_tool_use_config…