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.

apps/web/tests/message-actions.e2e.ts

@@ -126,7 +126,7 @@ describe('web e2e: message IconActions and clocks on settled history', () => {
 
   it.skipIf(MODE === 'record')('matches the conversation aria golden with IconActions and clocks', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-message-actions-aria'))
-    await page.getByRole('button', { name: 'Select model', exact: true })
+    await page.getByRole('button', { name: /^Select model, current/ })
       .waitFor({ timeout: 10_000 })
     // Keep a footer focused so opacity-hidden actions stay in the a11y tree
     // as an active/focused control during the capture.