refactor: extract request body for gemini_model.py.

[StarsExpress]

Refs issue #461 1st point: deduplicate body construction for code_puppy/gemini_model.py::GeminiModel.request(), GeminiModel.request_stream().

Extraction

Inside GeminiModel a shared method joins:

    def _build_request_body(
        self,
        system_instruction,
        contents,
        model_settings,
        model_request_parameters,
        *,
        streaming: bool,
    ) -> dict[str, Any]:
        body: dict[str, Any] = {"contents": contents}

        gen_config = self._build_generation_config(model_settings)
        if gen_config:
            body["generationConfig"] = gen_config

        if system_instruction:
            body["systemInstruction"] = system_instruction

        # Add tools.
        if model_request_parameters.function_tools:
            body["tools"] = self._build_tools(model_request_parameters.function_tools)

            if streaming:
                body["toolConfig"] = {
                    "functionCallingConfig": {
                        "mode": "AUTO",
                        "streamFunctionCallArguments": True,
                    }
                }

        return body

How GeminiModel.request() calls GeminiModel._build_request_body():

body = self._build_request_body(
            system_instruction=system_instruction,
            contents=contents,
            model_settings=model_settings,
            model_request_parameters=model_request_parameters,
            streaming=False,
        )

How GeminiModel.request_stream() calls GeminiModel._build_request_body():

body = self._build_request_body(
            system_instruction=system_instruction,
            contents=contents,
            model_settings=model_settings,
            model_request_parameters=model_request_parameters,
            streaming=True,
        )

Next step

After PR #899 gets approval and joins main, I will post another PR for #461 2nd point.