Fix OTel for built-in tools returning a list (e.g. Anthropic web search)

[DouweM]

Follow-up to #2954

Fixes this example reported on Slack:

from typing import TypedDict

import logfire
from pydantic_ai import Agent, WebSearchTool

logfire.configure()
logfire.instrument_pydantic_ai()


class Person(TypedDict):
    name: str
    age: int


agent = Agent(
    'anthropic:claude-sonnet-4-0',
    output_type=str | Person,
    builtin_tools=[WebSearchTool()],
    instructions="""
You are a helpful assistant that can find people online, and return their age, if you don't find it, explain me how I can find it.
""",
)

agent.run_sync("Search for Marcelo Trylesinski and return their age.")

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