[Bridge] Delivered progress to NexusHandler callbacks as OnProgress tasks.

[moetemp]

What changed?

This PR delivers progress to a NexusHandler callback that names a progress operation, as a sync Nexus task on the Worker's task queue.

• variantProgressDelivery returns deliverNexusHandlerProgress when progress_operation is set. The fold and backoff are the shared machine's.
• A sync OnProgressResponse gives the new receiver state. A refusal turns progress off, and retryable errors retry. No result counts against the circuit breaker.
• With behind set, the same progress goes again after callback.nexusHandler.progressBehindInterval (1 s), unless newer progress replaced it.
• callback.nexusHandler.receiverStateMaxSize caps the state at 8 KiB.

It replaces moetemp/temporal#44, ported onto Jetty's delivery seam.

Why?

One progress path serves both callback variants, and a slow follower can't hold back completions (DD-110). behind lets a Worker read a long backlog in pages that fit the request timeout (DD-112).

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

TestNexusHandlerProgressDelivery in progress_nexus_test.go.