feat: do not expect response for newCheckpoint

Igor Drozdovrequested to merge
id-dont-expect-response-for-new-checkpoint into main

What does this merge request do and why?

Currently, we send an empty response as a response to newCheckpoint grpc message. However, it blocks a server for a round-trip time because the server cannot proceed with the next token until it receives an empty response from the client.

The functionality is guarded by x-gitlab-unidirectional-streaming header and not a feature flag because we need to migrate every client that uses streaming one-by-one. An example for Web: [Agentic Chat] Do not respond to NewCheckpoint ... (gitlab-org/gitlab!197761 - merged)

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