Add Deep Research tool

### What is the problem this feature will solve?

The agent can answer targeted questions, but it cannot perform open-ended, multi-source investigation over statistical data and publications. Users need a way to ask broad research questions and get a synthesized, grounded answer that draws on multiple datasets and sources.

### What is the proposed feature or solution?

Add a **Deep Research** agent tool that decomposes a research question, gathers evidence across existing data/publication sources, and synthesizes a grounded, cited report.

Link: https://github.com/epam/ai-dial-deep-research-backend

TODO:
- [x] Create a basis for a new tool and configuration.
- [x] Provide streaming of `stages` and `attachments`.
- [x] Add saving of conversation between the Supreme Agent and the Deep Research to `state`.
- [x] New user messages should be forwarded to the Deep research, until it is completed. (Clarification flow) We need to discuss a specific mechanism.
- [x] Deep Research mode shall be activated exclusively by user clicking on a dedicated UI control (This needs to be implemented on the frontend side first.). This button forces the Supreme Agent to call the Deep Research tool (achievable via `tool_choice` parameter, see [DIAL API](https://dialx.ai/dial_api#operation/sendChatCompletionRequest)).

### What alternatives have you considered?

N/A