Pinned the Core that vendors the stream record envelope.

[moedash]

This PR pins the Core that vendors the stream record envelope and regenerates the Python protos from it.

What changed?

• temporalio/bridge/sdk-core moves to the Core layer that adds temporal/api/stream/v1/message.proto.
• poe gen-protos from that pin adds temporalio.api.stream.v1 with StreamRecord and StreamRecordKind. Nothing else changes, and nothing uses the module yet.

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

Why?

The stream interface puts one record on the wire for every provider, and that record is this proto. It reaches Python the usual way, through the Core's vendored api tree, so a later poe gen-protos from the pin stays correct. The interface itself is the next PR.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

The bridge builds and poe lint is clean. A second poe gen-protos leaves the tree as committed.