feat(ui): make a session that cannot send refuse to accept one

A default naming a route the Models page has since removed left the
composer saying 选择模型 while the input still accepted a message, which
then failed inside the adapter mid-turn.

`session.prompt` now refuses with `model-unavailable` before opening a
turn. That is the enforcement boundary: the method stays callable no
matter what a client disables. `session.models` reports the same fact as
`routable`, and ui-model pushes a block through the new
`ctx.conversation.blocks` registry so the bar renders the disabled
textarea it already renders without a workspace, carrying the blocker's
own reason. The push direction is forced — ui-model already depends on
ui-conversation, so ui-conversation cannot read it back.

The gate is `routable`, not "matches no advertised group": catalog
membership is advisory, so a route serving a model it stopped advertising
is missing from the groups yet perfectly usable, and `null` before the
first load never blocks so a slow Host cannot lock a working composer.

The scaffold gains a route-only adapter for fixture-less keyless
scenarios. Registering zero providers is a test artifact — every product
composition mounts one — and the goldens that froze the seat's fallback
label now show the model those scenarios actually route to.

packages/client/connection/src/client/fixture.ts

@@ -1975,6 +1975,9 @@ export function createFixtureApi(options: FixtureOptions = {}): ApiProxy {
       models: request => ok(request, {
         current: modelTargets.get(request.payload.sessionId)
           ?? { provider: 'deepseek-official', model: 'deepseek-v4-flash' },
+        // The fixture's routes all serve; a surface exercising the blocked
+        // posture drives it through its own stub.
+        routable: true,
         groups: fixtureModelGroups(),
         failures: [],
       }),

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

@@ -59,6 +59,7 @@ export class FakeApiClient implements IApiClient {
 
   onModels: (payload: unknown) => Promise<RpcResponse<SessionModels>> = () => Promise.resolve(ok({
     current: { provider: 'deepseek-official', model: 'deepseek-chat' },
+    routable: true,
     groups: [],
     failures: [],
   }))