feat(llm): interrogate a draft provider endpoint for its models

Once a pi-ai route became a declaration rather than a catalog lookup,
adding an OpenAI-compatible gateway meant knowing its model ids up
front. Most such endpoints publish that list at `GET /models`, but no
seam operation could ask: every one is keyed by a registered provider
route, and the provider being added has no route, no stored profile,
and no stored credential — the endpoint and key are values in a form.

Interrogation is therefore keyed by settings namespace, which a
configuration surface already holds from the configurable-provider
directory. `registerModelDiscovery` offers it per namespace,
`discoverModels` asks, and the request carries the draft itself. The
reply is candidates, not a catalog: every field but the id is optional
because most listings disclose nothing else, and adopting one is a
settings write like any other. Nothing here reads or writes settings or
credentials, so `settings.yaml` still decides what a route serves.

`llm.discoverModels` carries the same draft over the wire. Its apiKey is
the third and last payload a secret may ride, and it is never stored,
logged, or echoed; every refusal folds into `model-discovery-failed`,
naming the endpoint asked but never the credential offered.

The pi-ai side is a plain GET for OpenAI-compatible protocols only —
their listing shape is the one gateways, self-hosted servers, and the
official endpoints agree on. Others say so, sending the user to
hand-entry rather than reporting a guessed shape as an empty provider.
The reply is read under a four-megabyte ceiling held on the bytes
actually received, because the endpoint is a URL the user typed.

packages/client/runtime/tests/fake-api.ts

@@ -232,6 +232,7 @@ export class FakeApiClient implements IApiClient {
   readonly llm: IApiClient['llm'] = {
     providers: payload => this.record('llm.providers', payload, Promise.resolve(ok({ providers: [] }))),
     models: payload => this.record('llm.models', payload, Promise.resolve(ok({ groups: [], failures: [] }))),
+    discoverModels: payload => this.record('llm.discoverModels', payload, Promise.resolve(ok({ models: [] }))),
   }
 
   /** When true, streams never fire onOpen (misbehaving-carrier material for the handshake timeout guard). */