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/host/apiproxy/src/api/sessions.schema.ts

@@ -225,6 +225,7 @@ export const sessionModelsRequestSchema = z.object({
 /** session.models response value. */
 export const sessionModelsValueSchema = z.object({
   current: modelTargetSchema,
+  routable: z.boolean(),
   groups: z.array(modelProviderGroupSchema),
   failures: z.array(modelCatalogFailureSchema),
 }) satisfies z.ZodType<Wire<ResponseValue<'session.models'>>>

packages/host/apiproxy/src/api/sessions.ts

@@ -117,6 +117,15 @@ export interface ModelCatalogFailure {
 export interface SessionModels {
   /** Target selected for the session's next assembled step. */
   current: ModelTarget
+  /**
+   * Whether an adapter currently serves `current.provider`, and therefore
+   * whether this session can start a turn at all. Deliberately NOT derivable
+   * from `groups`: catalog membership is advisory, so a route serving a model
+   * it stopped advertising is absent from the groups yet perfectly usable,
+   * while a route whose adapter is gone can serve nothing. A surface that
+   * blocks input must read this rather than the groups.
+   */
+  routable: boolean
   /** Successfully loaded provider groups. */
   groups: ModelProviderGroup[]
   /** Provider-local failures; successful groups remain usable. */