Introduce tool_use_behavior on agents

[rm-openai]

Context

By default, the outputs of tools are sent to the LLM again. The LLM gets to read the outputs, and produce a new response. There are cases where this is not desired:

1. Every tool results in another round trip, and sometimes the output of the tool is enough.
2. If you force tool use (via model settings tool_choice=required), then the agent will just infinite loop.

This enables you to have different behavior, e.g. use the first tool output as the final output, or write a custom function to process tool results and potentially produce an output.

Test plan

Added new tests and ran existing tests
Also added examples.

Closes #117

[TimoVink]

Perhaps a try/except/rethrow around the call to the user-provided function with a useful error message?

Was just playing with this branch and even with this function it was stopping after 1 turn.

def custom_tool_use_behavior(context, results):
    return ToolsToFinalOutputResult(is_final_output=False, final_output=None)

... turns out it was a silly mistake on my part where I had forgotten to import ToolsToFinalOutputResults, but there were no obvious user-visible errors that I could find -- even with the openai.agents logger set to DEBUG. It just looked as-if the agent had decided it was done even though is_final_result was False.

[rm-openai]

oh weird! Yeah I'll add that. Thanks

[TimoVink]

This completely supports my use cases. Thanks!

Minor suggestion: In my case an agent's goal is to invoke one particular tool, but needs to use a few others tools to get there. I imagine this may be a fairly common usecase.

If you agree, there's two ways the library could make that usecase even easier to handle:
(1) Similar to the ModelSettings.tool_choice parameter, allow tool_use_behaviour to be a str that matches a tool name. And the behaviour would be "stop after the named tool has been called".
(2) Or, provide a factory function similar to the below:

def stop_after_tool_use(tool: str | FunctionTool) -> ToolsToFinalOutputFunction:
    if isinstance(tool, FunctionTool):
        tool = tool.name
    
    def custom_tool_use_behavior(context, results):
        for result in results:
            if tool == result.tool.name:
                return ToolsToFinalOutputResult(is_final_output=True, final_output=result.output)
        return ToolsToFinalOutputResult(is_final_output=False, final_output=None)

    return custom_tool_use_behavior

# Usage example:
@function_tool
def get_foo_id(user_input):
   ...

@function_tool
def get_bar_id(user_input):
    ...

@function_tool
def get_large_dataset(foo_id, bar_id):
    ...

agent = Agent(
    name="Foo Agent",
    instructions="Using your tools, retrieve the requested dataset.",
    tools=[get_foo_id, get_bar_id, get_large_dataset],
    tool_use_behavior=stop_after_tool_use(get_large_dataset),
)

[rm-openai]

good suggestion