Added a Nexus operation that consumes a stream through its channel.

[moedash]

This PR adds stream_consumer_operation, an async Nexus operation that consumes a stream and completes when the stream closes.

What changed?

• StreamConsumerOperation takes a StreamRef, folds the stream's records with a consume function, and returns the result. On start it registers a callback listener on the stream's notification channel, then reads from the start. Each delivery triggers a read up to the head. The close completes the operation through the caller's completion callback.
• channel_for defaults to temporalio.client.stream_channel, the same rule the server uses. Another rule can be passed for a store that notifies elsewhere.
• nexus_consumer_service is a standalone aiohttp host that serves the operation and receives the channel's callbacks. aiohttp comes in through the new streams-nexus extra.
• The consumer's changelog line. needs_native_provider gates the one case that needs native streams until the union.

Part of AI-198 (epic AI-37).

Why?

A caller often wants "run this when the stream is done" without holding a reader open. A Nexus operation fits: it's async, it survives restarts, and the caller's workflow just awaits it. Because the channel drives it, nobody polls. This PR is where the interface meets the channel, and it needs both below it.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

poe lint is clean, and uv lock --check accepts the lock. The unit cases pass with a client double, covering start, delivery, cancel, close, failure and the default channel rule. The live cases ran against a local server built from the channel server PRs, with a Nexus callback template and local callback addresses in its dynamic config. They cover a standalone service consuming an external stream through its channel, cancel unregistering the listener, and a worker-hosted consumer reached through the frontend, and they pass. The native case stays gated until the union.