Phase 1: Replicate fundamental features of Agentic Chat in Flow Registry
## Problem to solve Based on the PoC (https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/work_items/2150 and https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/merge_requests/5328), we need to introduce some fundamental functionalities in Flow Registry that are required to port the current Agentic Chat. Some of the PoC's changes are touching the core design of the Flow Registry, thus there is a possibility that these proposals are refined or changed due to the architectural direction (i.e. we can't just change for Agentic Chat as these components are shared in the other Flows). By this reason, it's practical to finish the implementation for the known issues at first, before continuing the PoC assessment. This phase 1 aims to replicate the core conversation feature of Agentic Chat, such as context expansion and response streaming. Please see the child and linked items for more details. After these issues have been resolved, we'll resume [the PoC assessment](https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/work_items/2150) and plan phase 2 for completing the migration.
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