Add simplified run() API for OpenAI Agents SDK integration

[danielmillerp]

• New run() method accepts standard Agent objects from OpenAI SDK
• Automatically converts ModelSettings and FunctionTool to serializable format
• Provides clean DX matching OpenAI Agents SDK with just task_id param
• Works in Temporal workflows, sync agents (FastACP), and standalone scripts

Example usage:
from agents import Agent, function_tool, ModelSettings

@function_tool
def my_tool(param: str) -> str:
    return f"Result: {param}"

agent = Agent(
    name="My Agent",
    instructions="Help users",
    model="gpt-4o",
    model_settings=ModelSettings(parallel_tool_calls=True),
    tools=[my_tool]
)

result = await adk.providers.openai.run(
    agent=agent,
    input="Hello",
    task_id=task_id
)