AsyncModels.generate_content crashes with cannot pickle '_asyncio.Future' when config is a GenerateContentConfig object containing an MCP ClientSession tool

### Environment

- google-genai: 2.9.0
- mcp: 1.28.1
- Python: 3.12.13

### The issue

Passing a live MCP ClientSession as a tool works when config is a dict, but the same settings passed as a GenerateContentConfig object raise TypeError: cannot pickle '_asyncio.Future' object.

In AsyncModels.generate_content, the config is deep-copied before MCP sessions are extracted — google/genai/models.py (2.9.0)

### Steps to reproduce

1. Create a live MCP ClientSession and pass it as a tool to aio.models.generate_content, with config as a GenerateContentConfig object:
config=types.GenerateContentConfig(tools=[session]).
2. The call raises TypeError: cannot pickle '_asyncio.Future' object before any network request.
3. Pass the same settings as a dict instead — config={"tools": [session]} — and it works (session is extracted, request proceeds normally).