[cboettig]

Closes #154. Part of #142 and #120 (shape 2 from #154).

Change

get_provider_for_model() gains one tier between declared prefixes and default_provider:

1. exact models (unchanged; nimbus qwen still wins)
2. declared model_prefixes (unchanged; ~ floating aliases, claude-)
3. new: an id containing / goes to namespaced_default_provider. That defaults to openrouter when configured. With no usable value it is a 400 naming the setting, never a fallback to default_provider.
4. bare ids go to default_provider (nrp, unchanged)

NRP serves no slash-namespaced ids. In the last 60 days of logs the only namespaced id routed to NRP was the #142 repro (google/gemini-3.7-flash), and it failed there. So no working route changes. reflection/…, meta/… and google/… now reach OpenRouter without a prefix edit.

GET /v1/models is filtered through the router, so it now lists OpenRouter's whole catalog, not only the declared vendors.

Not changed

• The proxy does not validate ids against the OpenRouter catalog (Routing ignores the model catalog the proxy already fetches #142 shape 1). OpenRouter's own 4xx for an id it doesn't serve is relayed, and the error now names the right backend. Catalog validation can layer on later.
• The served-model and upstream-provider logging from headless: archive matrix logs to S3; record the served model per call #148 and feat(logging): record the OpenRouter provider that served each call (upstream_provider) #151 is untouched.
• openrouter.model_prefixes stays in config.json. It is now redundant for routing but harmless.

Tests

66 pass. Two are new: unlisted vendors route to OpenRouter while bare, exact and claude- ids are unchanged; and with no namespaced fallback the id raises UnknownModelError and does not reach NRP. The discovery-listing test was updated for the intended change.