Pinned the channel Core on the external base and regenerated the protos.

[moetemp]

This PR pins the external-streams Core that carries notification channels and regenerates the Python protos from it.

What changed?

• temporalio/bridge/sdk-core points at the channel Core built on this branch's own Core base. .gitmodules now names moetemp/sdk-rust, since the old URL doesn't have the commit.
• The rest is what poe gen-protos writes from that pin: the notification/v1 package, the channel commands, events and failed causes, ChannelSubscriptionInfo, the five channel RPCs and their messages, the NotificationsReceived job and the two channel commands in the bridge protos. The pin forces the _visitor.py, services_generated.py and client_rpc_generated.rs changes too.

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

Why?

The channel surface in the next PR needs these types and RPCs. Keeping the pin on its own means that PR shows only hand-written code. The old external branch layered the api diff over the vendored protos with scripts/gen_stream_api_protos.py. That script isn't coming back, because this Core vendors the channel api itself and plain poe gen-protos covers it. There's no stream api in here yet.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

The bridge builds, and poe bridge-lint and poe lint are clean. The Nexus system generator runs again on this pin and gives back the committed output. The client, visitor and external stream suites pass on the dev server the fixtures start. Nothing calls the new types yet. That starts in the next PR.

[moetemp]

Replaced by #71 in the v4 series: one lineage through the channel and the interface, with Max's Option 7 prototype replayed onto it. The branch stays as a pin.