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/host/apiproxy/src/api/rpc.ts

@@ -70,6 +70,15 @@ export interface RpcErrorDetailsMap {
   'settings-conflict': { ns: string; expected: number; actual: number }
   /** A credential write was refused (read-only shadowing layer or storage failure); the message is the seam's own text. */
   'credential-rejected': { ref: string }
+  /**
+   * Interrogating a draft provider endpoint did not produce a model listing:
+   * no adapter family serves the namespace, the protocol has no listing this
+   * build can read, or the endpoint was unreachable, refused the credential,
+   * or answered with something else. The message is the adapter's own text —
+   * it is what the form shows before falling back to hand-entry — and the
+   * details name the endpoint asked, never the credential offered.
+   */
+  'model-discovery-failed': { settingsNs: string; baseURL: string }
   'title-invalid': { sessionId: SessionId }
   'fork-unavailable': { sessionId: SessionId }
   'subagent-parent-unavailable': { parentSessionId: SessionId }