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

[moetemp]

What changed?

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

• deliverNexusHandler dispatches progress_operation with an OnProgressRequest that carries the saved receiver state. Fold and backoff are the Nexus variant's code.
• A sync OnProgressResponse gives the new state. A refusal turns progress off, and retryable errors retry.
• No result counts against the circuit breaker, so a Worker that's down holds back no completion.
• With behind set, the same progress goes again at once, unless newer progress replaced it.
• callback.nexusHandler.receiverStateMaxSize caps the state at 8 KiB.

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

TestProgressDeliveryNexusHandler in progress_test.go.

[moetemp]

Replaced by Bridge v2 #62. Bridge was re-cut onto Jetty (stream wake-ups in one namespace) with no Ford content. The same-namespace notifier parts moved to Jetty, and the Nexus parts are in Bridge v2.