Update default output to string handler in `ToolInvoker` to serialize before using `json.dumps`

[sjrl]

In the ToolInvoker we have an option called convert_result_to_json_string which does this currently

haystack/haystack/components/tools/tool_invoker.py

Lines 215 to 227 in 9ae7da8

	def _default_output_to_string_handler(self, result: Any) -> str:
	"""
	Default handler for converting a tool result to a string.
	:param result: The tool result to convert to a string.
	:returns: The converted tool result as a string.
	"""
	if self.convert_result_to_json_string:
	# We disable ensure_ascii so special chars like emojis are not converted
	tool_result_str = json.dumps(result, ensure_ascii=False)
	else:
	tool_result_str = str(result)
	return tool_result_str

I believe it would be better if we followed the implementation of coerce_tag_value

haystack/haystack/tracing/utils.py

Lines 31 to 39 in 9ae7da8

	try:
	# do that with-in try-except because who knows what kind of objects are being passed
	serializable = _serializable_value(value)
	return json.dumps(serializable)
	except Exception as error:
	logger.debug("Failed to coerce tag value to string: {error}", error=error)
	# Our last resort is to convert the value to a string
	return str(value)

which first calls _serializable_value which iteratively goes through result and tries to serialize the value using to_dict if it's available.

This way Haystack native items like ChatMessage and Documents are better represented in a JSON format than by directly calling json.dumps on them.