evals: AgentConfig.from_agent raises TypeError for ADK agents using toolsets (e.g. McpToolset) #6861

Environment

• google-cloud-aiplatform 1.156.0 (also reproduces on current main)
• google-adk 1.31.1, Python 3.12

Steps to reproduce

Run client-side eval inference over an ADK agent whose tools include any ADK toolset (a BaseToolset such as McpToolset or OpenAPIToolset):

from google.adk.agents import Agent
from google.adk.tools.mcp_tool.mcp_toolset import McpToolset
from google.adk.tools.mcp_tool.mcp_session_manager import StreamableHTTPConnectionParams
import vertexai

agent = Agent(
    name="repro",
    model="gemini-2.5-flash",
    instruction="...",
    tools=[McpToolset(connection_params=StreamableHTTPConnectionParams(url="https://example.com/mcp"))],
)
client = vertexai.Client(project="...", location="...")
client.evals.run_inference(src=dataset, agent=agent)  # raises

Actual behavior

File ".../vertexai/_genai/types/evals.py", line 128, in from_agent
    tools=AgentConfig._get_tool_declarations_from_agent(agent),
File ".../vertexai/_genai/types/evals.py", line ~100, in _get_tool_declarations_from_agent
    genai_types.FunctionDeclaration.from_callable_with_api_option(callable=tool)
File ".../google/genai/types.py", line 4405, in from_callable_with_api_option
    for name, param in inspect.signature(callable).parameters.items():
TypeError: <google.adk.tools.mcp_tool.mcp_toolset.McpToolset object at 0x...> is not a callable object

Analysis

_get_tool_declarations_from_agent handles ADK tools exposing _get_declaration() (since the from __future__ import annotations fix), but ADK toolsets have no _get_declaration and are not plain callables — they expose their tools only asynchronously via get_tools(readonly_context). They fall through to the inspect.signature() path and raise. Any MCP-tooled ADK agent is uninspectable for eval inference as a result (this breaks agents-cli eval generate for such agents too).

Proposed fix

Skip toolsets in the extraction loop (declarations are metadata; toolset tools resolve only at runtime). PR incoming for both vertexai/_genai/types/evals.py and agentplatform/_genai/types/evals.py.