fix(chat): thread the steered blocks through the whole composer chain

`onSteer` widened to `(text, blocks?)` in `MessageInput` and in the panel's
handler, but `ConversationShell` and `ChatInput` — the two layers the prop
actually travels through — still declared the one-argument form. The optional
second parameter keeps that assignable, so tsc reports nothing and the leak
would only appear the day either layer wraps the callback instead of
forwarding it.

Also lock down the two claims the change rests on: the text-only steer's wire
shape is now asserted by exact equality rather than field probes, and the
enqueue fallback is asserted to carry the image block, not just the prose.