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/snapshots/markdown-inline-code-links/ui.expected.md

@@ -33,8 +33,8 @@
 - button "Commands":
   - img
 - 'button "Access mode, current: Workspace Write"': Workspace Write
-- button "Select model":
-  - text: Select model
+- button "Select model, current DeepSeek-V4-Flash":
+  - text: DeepSeek-V4-Flash
   - img
 - button "Send message" [disabled]
 - text: 1 turns · 1 steps LLM {{duration}} Input 0 tok · Output 0 tok