Handle tool errors better in tool registry

[fswair]

The built-in errors comes from _function_schema origin, related lines, 100, 238, 245.

The issue is basically comes from built-in functions. If we can register a built-in function as a tool, it raise errors.

See main issue for details: #2251

[fswair]

Tests:

import pytest


def test_builtin_tool_registry():
    """
    Test that built-in functions can be registered as tools.
    """
    from pydantic_ai import Agent, Tool
    from pydantic_ai.exceptions import UserError
    from pydantic_ai.models.test import TestModel
    
    with pytest.raises(UserError):
        Agent(TestModel(), tools=[max])
    
    with pytest.raises(UserError):
        agent = Agent(TestModel())
        agent.tool_plain(min)
    
    agent = Agent(TestModel(), tools=[print])
    assert 'print' in agent._function_toolset.tools
    
    agent._function_toolset.tools.pop('print', None)
    
    agent._function_toolset.add_tool(Tool(print))
    assert 'print' in agent._function_toolset.tools