### Check for existing issues

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

### What happened?

On AWS Bedrock Converse, a request that contains no tools but sets parallel_tool_calls (or tool_choice) fails with a 400 from Bedrock. Agent frameworks (e.g. n8n) default parallel_tool_calls=false on every turn, including turns with no tools, so ordinary toolless chat completions 400.

Two separate leaks cause this: 1. tool_choice is only removed from the request inside if len(bedrock_tools) > 0, so on a toolless request it leaks into inferenceConfig → Bedrock rejects it. 2. The parallel_tool_calls-derived disable_parallel_tool_use block is injected into additionalModelRequestFields gated only on model capability, with no tools-presence check → a tool_choice block is sent with no toolConfig → 400.

Expected: toolless requests should not emit toolConfig, tool_choice, or the parallel-tool-use config; requests with tools keep current behavior.

### Steps to Reproduce

1. Configure any Bedrock Claude model that advertises supports_parallel_tool_use_config (e.g. bedrock/us.anthropic.claude-sonnet-...). 2. litellm.completion(model=..., messages=[{"…