Update C bridge replace_client to allow for heartbeat worker creation

[yuandrew]

What was changed

Enter the tokio runtime context for replace_client

Why?

This is now needed because for worker heartbeating we can spawn a task to create a new nexus-only worker to heartbeat, which requires being in a tokio context.

Checklist

1. Closes

2. How was this tested:
   Fixes test with dotnet update I'm trying to make

3. Any docs updates needed?

---

Note

Enter the Tokio runtime context before calling replace_client in crates/sdk-core-c-bridge/src/worker.rs.

^{Written by Cursor Bugbot for commit a8788e1. This will update automatically on new commits. Configure here.}