Tool calling crashes with NameError: is_mcp_config on base install (without mcp extra)

Describe the bug

When the optional mcp extra is not installed (i.e. a plain pip install aisuite), passing any tools= argument to client.chat.completions.create(...) raises NameError: name 'is_mcp_config' is not defined. This breaks all tool / function calling for every provider on a base install — even plain OpenAI-style function tools that have nothing to do with MCP.

Root cause

In aisuite/client.py, is_mcp_config is imported in the same try block as the heavy mcp dependency:

# aisuite/client.py
try:
    from .mcp.config import is_mcp_config   # line 15
    from .mcp.client import MCPClient        # line 16  <-- fails if `mcp` not installed
    MCP_AVAILABLE = True
except ImportError:
    MCP_AVAILABLE = False

If from .mcp.client import MCPClient raises ImportError (because the mcp package isn't installed), the whole block is abandoned, so is_mcp_config is never bound and MCP_AVAILABLE = False.

But _process_mcp_configs still calls is_mcp_config in exactly that case:

        if not MCP_AVAILABLE:
            # If MCP not installed, check if user is trying to use it
            if any(is_mcp_config(tool) for tool in tools if isinstance(tool, dict)):  # line 174  NameError
                raise ImportError(...)
            return tools, []

So the not MCP_AVAILABLE guard — the branch specifically meant to handle "mcp isn't installed" — itself references a name that only exists when mcp is installed.

To reproduce

pip install aisuite openai   # base install, NO mcp extra

import aisuite as ai

client = ai.Client(provider_configs={"openai": {"api_key": "..."}})
client.chat.completions.create(
    model="openai:gpt-4o-mini",
    messages=[{"role": "user", "content": "What's the weather in Paris?"}],
    tools=[{
        "type": "function",
        "function": {
            "name": "get_weather",
            "description": "Get the weather for a city",
            "parameters": {
                "type": "object",
                "properties": {"city": {"type": "string"}},
                "required": ["city"],
            },
        },
    }],
)

File "aisuite/client.py", line 174, in _process_mcp_configs
    if any(is_mcp_config(tool) for tool in tools if isinstance(tool, dict)):
NameError: name 'is_mcp_config' is not defined

Expected behavior

Plain function/tool calling should work without the mcp extra. Only the actual MCP code paths (MCPClient) should require the mcp package.

Suggested fix

is_mcp_config is a trivial pure dict check with no dependency on the mcp package, so it should not be gated behind the MCP import. Either import it unconditionally:

from .mcp.config import is_mcp_config  # no external deps

try:
    from .mcp.client import MCPClient
    MCP_AVAILABLE = True
except ImportError:
    MCP_AVAILABLE = False

or define a local fallback in the except branch.

Workaround

Install the mcp extra: pip install 'aisuite[mcp]' (this binds is_mcp_config, so tool calling works).

Environment

• aisuite installed from main
• Python 3.13, Windows
• Reproduced while validating a new provider's tool-calling support against a base install.