[Bridge] Regenerated for progress to NexusHandler callbacks.

[moetemp]

This PR regenerates api-go for the NexusHandler progress fields.

What changed?

• proto/api points at api U5a 6b7a4aca.
• The new code is Callback.NexusHandler.ProgressOperation, OnProgressRequest.OperationToken and ReceiverState, OnProgressResponse.ReceiverState and Behind, and OnCompleteRequest.ReceiverState, with the OpenAPI copies.
• Please note that the submodule URL still names moetemp/api, as on N4. It is a release blocker, with the server replace.

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

Why?

The server takes the API through api-go (DD-109, DD-112).

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

go build ./... and the proxy tests pass.