### 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 LiteLLM Proxy's `/responses` endpoint with a client that sends both an `instructions` field and `developer` role messages (e.g., Codex CLI), LiteLLM generates multiple `system` messages in the final chat completion request. Non-OpenAI backends (e.g., Qwen, Llama) that only accept a single system message at the beginning will reject the request with a 400 error: `System message must be at the beginning.`

## Root Cause

Two separate code paths each produce a `system` message, and they are never merged:

1. `LiteLLMCompletionResponsesConfig.transform_instructions_to_system_message()` in `transformation.py` converts the `instructions` field into `messages[0]` as a `system` role. 2. `map_developer_role_to_system_role()` in `base_utils.py` iterates over all messages and converts each `developer` role one-by-one into additional `system` messages.

The result is a message list like: ``` system (from instructions) system (from developer message 1) system (from developer message 2) ... user user ```

This is invalid for an…