docs(ai-transport): add token streaming example, reframe append example

Add a runnable AI Transport token streaming example that uses the AI
Transport SDK (v0.3.0), mirroring the Core SDK getting started: the agent
flow (createAgentSession, createRun, start, loadConversation, pipe, end)
and the client hooks (ClientSessionProvider, useView, useClientSession).
The model is mocked so it runs without a key, but emits the same
UIMessageChunk stream a production integration produces.

Move the former ai-transport-message-per-response example to
pub-sub-message-append under the Pub/Sub product, since it uses raw
pub/sub appendMessage rather than the SDK. Reframe its copy to point to
the token streaming example as the recommended approach for AI.

Remove the ai-transport-message-per-token example.

Wire up Sandpack and workspace dependencies (@ably/ai-transport, ai, bump
ably to 2.23 for the SDK peer) and bump the AI Transport SDK version
registry to 0.3. Update token-streaming and ai-chatbot pricing
meta descriptions that referenced the retired pattern names.

Author: rainbowFi