feat(ai-sdk): neutral queue/broadcast adapter seam (configureAiQueue)

[suleimansh]

What

Resolves the last @rudderjs/* coupling in the @gemstack/ai-sdk engine. agent.queue('...').send() and .broadcast(channel) no longer dynamically import @rudderjs/queue or @rudderjs/broadcast. Instead, the host registers a neutral adapter once at startup, mirroring the existing cache-adapter.ts / storage-adapter.ts seams.

New public API

configureAiQueue({ dispatch, broadcast }) registers the adapters and returns a restore function (handy in tests):

configureAiQueue({
  dispatch:  (fn, opts) => myQueue.push(fn, opts),
  broadcast: (channel, event, data) => myBus.emit(channel, event, data),
})

New exports from @gemstack/ai-sdk:

• configureAiQueue (function)
• QueueDispatch, QueueBroadcast (types, in new queue-adapter.ts)
• BroadcastOptions (type)

Behavior of the defaults

When no adapter is registered, the defaults no longer try to import @rudderjs/*. They fail loudly with a clear "configure it" message instead:

• agent.queue() throws: "agent.queue() needs a queue adapter. Register one at startup with configureAiQueue({ dispatch })."
• .broadcast() throws: ".broadcast() needs a broadcast adapter. Register one with configureAiQueue({ broadcast })."

Scope

This is the engine half. A follow-up @rudderjs/ai PR will call configureAiQueue in AiProvider.boot (wiring @rudderjs/queue's dispatch and @rudderjs/broadcast's broadcast), so Rudder apps keep working with no app change. This PR does not touch the Rudder repo.

Verification

• pnpm --filter @gemstack/ai-sdk build is clean.
• pnpm --filter @gemstack/ai-sdk test passes (902 tests).
• grep -rn '@rudderjs' packages/ai-sdk/src (excluding tests) is now empty.

Closes #48