[Go] Baidu/ERNIE: system prompt kept inside messages instead of top-level system field

## Description
In the Go Baidu/ERNIE driver (`internal/entity/models/baidu.go`), the system prompt is kept inside the `messages` array as `role:"system"` for both `ChatWithMessages` (L70) and `ChatStreamlyWithSender` (L254).

This diverges from the Python `BaiduYiyanChat` driver, which extracts the system message into a separate top-level `system=` parameter (`rag/llm/chat_model.py:1203`).

## Impact
Older ERNIE / Qianfan API versions reject a `system` role inside `messages` and require the separate `system` field. On those versions the Go driver may error out or silently mis-handle the system prompt.

## Expected behavior
Extract the system message(s) into a top-level `system` field (align with Python), or explicitly document/confirm that the targeted ERNIE version accepts an in-message `system` role.

## References
- `internal/entity/models/baidu.go:70`, `:254`
- Python (reference): `rag/llm/chat_model.py:1203`

Start with internal/entity/models/baidu.go at ChatWithMessages (L70) and ChatStreamlyWithSender (L254), then compare the system-message handling with rag/llm/chat_model.py:1203. Verify the behavior against the targeted ERNIE API expectations; done means both Go paths handle system prompts in the supported format without breaking ordinary messages.

Written by the indexing model from the issue text.