feat: add OpenAI Responses API provider with intelligent routing for GPT-5/o-series models

[82deutschmark]

This PR adds direct OpenAI Responses API support for modern reasoning models.

Key features:

• OpenAIProvider class - Direct OpenAI provider using /v1/responses endpoint
• Intelligent routing - GPT-5 and o-series models (o3, o4) automatically route to OpenAI directly
• Reasoning defaults - Automatically sets reasoning.effort='medium' and reasoning.summary='detailed' for reasoning models
• Model name normalization - Handles 'openai/gpt-4o' style namespaces gracefully

Why this matters:
OpenRouter's Responses API proxy returns empty output for GPT-5 and o-series models. This PR ensures these models work correctly by routing them directly to OpenAI when OPENAI_API_KEY is available.

Battle-tested in production.