Map OpenRouter ids to each provider's native model names

[TheCoder30ec4]

Motivation

Routes come back as OpenRouter ids such as anthropic/claude-opus-5.5. Users on Option A call the provider directly, and native names can differ (Anthropic's API uses claude-opus-5-5).

Proposal

Add router.native_model_id(model_id). Possible sources, in order:

1. A field in OpenRouter's model data, if one exists (check canonical_slug and hugging_face_id)
2. A small per-provider rule: drop the prefix, then provider-specific fixes (e.g. Anthropic . to -)
3. A user override: Router(..., native_ids={"anthropic/claude-opus-5.5": "claude-opus-5-5"})

Done when

• Mapping verified against real provider model lists for OpenAI, Anthropic and Google
• Documented, with unknown cases returning the input unchanged