feat: add MCPServerManager for safely managing server lifecycle

[seratch]

This pull request adds MCPServerManager to manage MCP server lifecycles safely (including optional parallel connects via worker tasks), expands guidance in the class docstring, and introduces a FastAPI manager example with an inactive server to demonstrate failure handling. Developers can use the new manager utility this way:

from contextlib import asynccontextmanager

from fastapi import FastAPI
from pydantic import BaseModel

from agents import Agent, Runner
from agents.mcp import MCPServerManager, MCPServerStreamableHttp

@asynccontextmanager
async def lifespan(app: FastAPI):
    async with MCPServerManager(
        servers=[
            MCPServerStreamableHttp({"url": 'http://localhost:8001/mcp'}),
            MCPServerStreamableHttp({"url": 'http://localhost:8002/mcp'}),
        ],
        connect_in_parallel=True,
    ) as manager:
        app.state.mcp_manager = manager
        yield

app = FastAPI(lifespan=lifespan)

class RunRequest(BaseModel):
    input: str

@app.post("/agent")
async def run_agent(req: RunRequest) -> dict[str, object]:
    agent = Agent(
        name="Test Agent",
        instructions="Use the MCP tools when needed.",
        mcp_servers= app.state.mcp_manager.active_servers,
    )
    result = await Runner.run(starting_agent=agent, input=req.input)
    return {"output": result.final_output}

This change keeps server.cleanup() intact and adds a small guard to catch asyncio.CancelledError during cleanup, preventing task cancellation from bubbling up and taking down the app.

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