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/tests/client-handler.spec.ts

@@ -112,6 +112,7 @@ function scriptedApi(overrides: {
     llm: {
       providers: r => ok(r, { providers: [] }),
       models: r => ok(r, { groups: [], failures: [] }),
+      discoverModels: err,
       ...overrides.llm,
     },
     events: { mux: () => empty<MuxFrame>(), host: () => empty<HostFrame>(), ...overrides.events },
@@ -694,6 +695,7 @@ describe('config unary surface', () => {
       llm: {
         providers: record('llm.providers', r => ok(r, { providers: [providerRow] })),
         models: record('llm.models', r => ok(r, { groups: [group], failures: [] })),
+        discoverModels: record('llm.discoverModels', r => ok(r, { models: [{ id: 'acme-large', contextWindow: 65536 }] })),
       },
     })
     const c = client(api)
@@ -719,16 +721,31 @@ describe('config unary surface', () => {
     expect(providers.result).toEqual({ ok: true, value: { providers: [providerRow] } })
     const models = await c.llm.models({})
     expect(models.result).toEqual({ ok: true, value: { groups: [group], failures: [] } })
+    const discovered = await c.llm.discoverModels({
+      settingsNs: 'llm-pi-ai',
+      baseURL: 'https://gateway.acme.example/v1',
+      api: 'openai-completions',
+      apiKey: 'probe-key',
+    })
+    expect(discovered.result).toEqual({ ok: true, value: { models: [{ id: 'acme-large', contextWindow: 65536 }] } })
 
     expect(seen.map(call => call.method)).toEqual([
       'settings.describe', 'settings.openDocument', 'settings.update', 'settings.replace', 'settings.mutate',
       'credentials.describe', 'credentials.set', 'credentials.unset',
-      'llm.providers', 'llm.models',
+      'llm.providers', 'llm.models', 'llm.discoverModels',
     ])
     expect(seen[2]?.payload).toEqual({ ns: 'llm-deepseek', patch: { baseURL: 'https://next' } })
     expect(seen[4]?.payload)
       .toEqual({ ns: 'llm-deepseek', ops: [{ op: 'unset', path: ['baseURL'] }], expectedRevision: 0 })
     expect(seen[6]?.payload).toEqual({ ref: 'OPENAI_API_KEY', value: 'sk-x' })
+    // The draft crosses whole, credential included: the host needs it for this
+    // one interrogation and stores none of it.
+    expect(seen[10]?.payload).toEqual({
+      settingsNs: 'llm-pi-ai',
+      baseURL: 'https://gateway.acme.example/v1',
+      api: 'openai-completions',
+      apiKey: 'probe-key',
+    })
   })
 
   it('rejects an invalid credential reference name at the carrier boundary', async () => {