feat(channels): stream outbound messages

[cjol]

This PR adds outbound message streaming to @cloudflare/channels, including native Slack and Telegram streaming, composite routing, and an AI SDK adapter.

Why

• Channels currently wait for a complete model response before delivering anything, even when the destination supports progressive rendering.
• Streaming needs transport-neutral chunks because provider-specific payloads do not compose across Slack, Telegram, fallback routes, and fanout routes.
• A Channel owns stream consumption and terminal provider calls. This ensures an interrupted generation still finalizes its provider message instead of leaving a draft or streaming message unfinished.
• Channels that cannot stream collect the text and use ordinary delivery, so callers use one Host API regardless of transport capability.
• Slack cannot stream at the top level of an ordinary channel. The adapter collects and posts one ordinary message there, while threads and direct messages use Slack's native streaming methods.

Public API Surface

Symbol	Kind	Notes
ChannelChunk	Type	Neutral text, reasoning, tool, and source stream chunks
ChannelChunkSource	Type	Normalized ReadableStream<ChannelChunk> input
ChannelDeliveryOptions	Type	Wraps caller-owned delivery context
ChannelStreamOptions	Type	Supplies an optional title and delivery context
Channel.stream	Optional method	Lets a Channel consume and finalize a streamed answer
OutboundResolver.stream	Method	Recursively streams through composite Channels
ChannelHost.stream	Method	Resolves a destination and streams or collects before delivery
consumeChunks	Function	Finalizes exactly once after normal or interrupted consumption
ChunkConsumer	Type	Defines chunk and finalization callbacks
StreamOutcome	Type	Reports normal or interrupted stream completion
toChannelChunks	Function	Maps an AI SDK fullStream to Channel chunks

DeliveryResult now permits a reference on its uncertain arm.

deliver now takes ChannelDeliveryOptions as its third argument instead of a bare ChannelDeliveryContext.

Architectural Changes

ReadableStream<ChannelChunk>
             |
        ChannelHost
             |
       +-----+------+
       |            |
 Channel.stream   collect text
       |            |
 native provider  Channel.deliver

Composite Channels preserve the same contract:

• fanout tees the source so each destination receives its own stream.
• fallback retains consumed chunks and replays them after a confirmed failure.
• uncertain stops fallback to avoid potentially duplicating partial delivery.

Code Changes

• Slack uses chat.startStream, chat.appendStream, and chat.stopStream for threads and direct messages. Top-level channel streams collect into chat.postMessage.
• Telegram uses sendMessageDraft for private-chat previews and always finishes with sendMessage, including after an interrupted generation.
• The Host collects text for Channels without native streaming support and uses TextSegmentJoiner to preserve semantic spacing around tool and metadata boundaries.
• The AI SDK adapter maps fullStream parts into neutral chunks and turns abort or error parts into abnormal stream completion.
• Live fixtures independently observe Telegram, Slack top-level, Slack threads, and email through one shared provider matrix.

Compatibility

The third argument to deliver changed from:

ChannelDeliveryContext

to:

ChannelDeliveryOptions

Callers passing a delivery ID must wrap it:

host.deliver(surface, message, {
  delivery: { deliveryId }
});

---

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🦋 Changeset detected

Latest commit: cc42f11

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@cloudflare/channels	Minor

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