Make shared session possible for MCP tool

[ekzhu]

Resolves #6232, #6198

This PR introduces an optional parameter session to mcp_server_tools to support reuse of the same session.

import asyncio

from autogen_agentchat.agents import AssistantAgent
from autogen_agentchat.conditions import TextMentionTermination
from autogen_agentchat.teams import RoundRobinGroupChat
from autogen_agentchat.ui import Console
from autogen_ext.models.openai import OpenAIChatCompletionClient
from autogen_ext.tools.mcp import StdioServerParams, create_mcp_server_session, mcp_server_tools


async def main() -> None:
    model_client = OpenAIChatCompletionClient(model="gpt-4o", parallel_tool_calls=False)  # type: ignore
    params = StdioServerParams(
        command="npx",
        args=["@playwright/mcp@latest"],
        read_timeout_seconds=60,
    )
    async with create_mcp_server_session(params) as session:
        await session.initialize()
        tools = await mcp_server_tools(server_params=params, session=session)
        print(f"Tools: {[tool.name for tool in tools]}")

        agent = AssistantAgent(
            name="Assistant",
            model_client=model_client,
            tools=tools,  # type: ignore
        )

        termination = TextMentionTermination("TERMINATE")
        team = RoundRobinGroupChat([agent], termination_condition=termination)
        await Console(
            team.run_stream(
                task="Go to https://ekzhu.com/, visit the first link in the page, then tell me about the linked page."
            )
        )


asyncio.run(main())

Based on discussion in this thread: #6284, we will consider serialization and deserialization of MCP server tools when used in this manner in a separate issue.

This PR also replaces the json_schema_to_pydantic dependency with built-in utils.

[codecov]

Codecov Report

Attention: Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.

Project coverage is 77.56%. Comparing base (844de21) to head (dd34581).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ges/autogen-ext/src/autogen_ext/tools/mcp/_base.py	90.90%	2 Missing ⚠️
...n-core/src/autogen_core/utils/_json_to_pydantic.py	75.00%	1 Missing ⚠️
.../autogen-ext/src/autogen_ext/tools/mcp/_factory.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6312      +/-   ##
==========================================
+ Coverage   77.49%   77.56%   +0.07%     
==========================================
  Files         202      202              
  Lines       14742    14754      +12     
==========================================
+ Hits        11424    11444      +20     
+ Misses       3318     3310       -8     

Flag	Coverage Δ
unittests	77.56% <90.24%> (+0.07%)	⬆️

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