Fix Custom Final Answer Tool for ToolCallingAgent

[NoRaincheck]

An attempt at fixing #1254

It forces the ToolCallingAgent to always execute the final tool, so that custom final tools will run as intended.

If appropriate please feel free to make changes directly.

Tested locally

[albertvillanova]

Thanks for the fix, @HeardACat.

I am reviewing it.

[albertvillanova]

Thanks.

I added a test and fixed the implementation to avoid calling the final_answer tool with non-JSON-serializable arguments (image, audio), which raise an error:

TypeError: Object of type AgentImage is not JSON serializable
https://github.com/huggingface/smolagents/actions/runs/14885689071/job/41804563090?pr=1255

FAILED tests/test_monitoring.py::MonitoringTester::test_streaming_agent_image_output - TypeError: Object of type AgentImage is not JSON serializable

/opt/hostedtoolcache/Python/3.10.17/x64/lib/python3.10/site-packages/smolagents/agents.py:1201: in execute_tool_call
    f"Error executing tool '{tool_name}' with arguments {json.dumps(arguments)}: {type(e).__name__}: {e}\n"
/opt/hostedtoolcache/Python/3.10.17/x64/lib/python3.10/json/__init__.py:231: in dumps
    return _default_encoder.encode(obj)
/opt/hostedtoolcache/Python/3.10.17/x64/lib/python3.10/json/encoder.py:199: in encode
    chunks = self.iterencode(o, _one_shot=True)
/opt/hostedtoolcache/Python/3.10.17/x64/lib/python3.10/json/encoder.py:257: in iterencode
    return _iterencode(o, 0)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <json.encoder.JSONEncoder object at 0x7f5f89fdbf70>
o = <smolagents.agent_types.AgentImage image mode= size=0x0 at 0x7F5E26E18940>

>       raise TypeError(f'Object of type {o.__class__.__name__} '
                        f'is not JSON serializable')
E       TypeError: Object of type AgentImage is not JSON serializable

/opt/hostedtoolcache/Python/3.10.17/x64/lib/python3.10/json/encoder.py:179: TypeError