[fix] OpenResponses: skip empty reasoning block to avoid breaking non-reasoning models

[NIK-TIGER-BILL]

Problem

_set_reasoning_request_param() in models/openai/responses.py unconditionally sets:

base_params["reasoning"] = self.reasoning or {}

This means every request carries "reasoning": {} even when no reasoning is configured. Models that do not support reasoning (e.g. Gemini 3.1 Pro Preview via OpenRouter) reject such payloads (closes #6945):

Error: reasoning is not supported for model google/gemini-3.1-pro-preview

Fix

Build the reasoning dict from self.reasoning + any reasoning_effort / reasoning_summary overrides, then only include it in base_params when the result is non-empty:

reasoning = dict(self.reasoning) if self.reasoning else {}
if self.reasoning_effort:
    reasoning["effort"] = self.reasoning_effort
if self.reasoning_summary:
    reasoning["summary"] = self.reasoning_summary

if reasoning:  # skip entirely when empty
    base_params["reasoning"] = reasoning

Closes #6945