Gemini could not understand multiple system_message when work with openai SDK

### What happened?

**Describe the bug**
This BUG is related by #6116.
When anthropic and openAI with openAI SDK case, Multiple system messages are working.

However, Gemini case, it could not understand multiple system message, Just understand last SystemMessage.

**To Reproduce**
Here is test code.
```python
clients =  [
    anthropic_client,
    openai_client,
    anthropic_openai_client,
    gemini_openai_client,
]

from autogen_agentchat.agents import AssistantAgent
from autogen_core.models import (
    SystemMessage,
    UserMessage,
)


messages = [
    SystemMessage(content="When you say anything Start with 'FOO'"),
    SystemMessage(content="When you say anything End with 'BAR'"),
    UserMessage(content="Just say '.'", source="user"),
]

for client in clients:
    client = client()
    print(f'=====[{client.model_info.get("family")}]=====')
    # try:
    text = asyncio.run(client.create(messages=messages))

    print(text.content)
    # except Exception as e:
    #     print("ERROR", e, end=" ")
    #     print(f"Error with {client.model_info.get('family')}")
    print("=====================================")
```

Output(When fix #6116 haha)
```
=====[claude-3.7-sonnet]=====
FOO.BAR
=====================================
=====[gpt-4o]=====
FOO. BAR
=====================================
=====[claude-3.7-sonnet]=====
FOO.BAR
=====================================
=====[gemini-2.0-flash]=====
.BAR

=====================================
```

You can see it. Gemini only says BAR.
Maybe.. it's just Gemini could not understand my system message?

Let's test it.
```python
messages = [
#     SystemMessage(content="When you say anything Start with 'FOO'"),
#     SystemMessage(content="When you say anything End with 'BAR'"),
    SystemMessage(content="When you say anything Start with 'FOO'\nWhen you say anything End with 'BAR'"),
    UserMessage(content="Just say '.'", source="user"),
]

for client in clients:
    client = client()
    print(f'=====[{client.model_info.get("family")}]=====')
    # try:
    text = asyncio.run(client.create(messages=messages))

    print(text.content)
    # except Exception as e:
    #     print("ERROR", e, end=" ")
    #     print(f"Error with {client.model_info.get('family')}")
    print("=====================================")
```
Output(When fix #6116 haha)
```
=====[claude-3.7-sonnet]=====
FOO.BAR
=====================================
=====[gpt-4o]=====
FOO.BAR
=====================================
=====[claude-3.7-sonnet]=====
FOO.BAR
=====================================
=====[gemini-2.0-flash]=====
FOO.BAR

=====================================
```
Nah~
We could check the gemini's openSDK aware codes has bug in there.


**Expected behavior**
I do not check the what code make that issue now.


### Which packages was the bug in?

Python AgentChat (autogen-agentchat>=0.4.0), Python Extensions (autogen-ext)

### AutoGen library version.

Python dev (main branch)

### Other library version.

OpenAI SDK

### Model used

gemini-2.0-flash

### Model provider

Google Gemini

### Other model provider

_No response_

### Python version

None

### .NET version

None

### Operating system

None

Labels: needs-triage
State: closed