This commit refactors the deep research module (deep_research.py), with
the following major improvements: The complex thinking and retrieval
logic has been broken down into multiple independent private methods,
enhancing code readability and maintainability. Static methods and class
methods have been introduced to simplify the logic for tag processing.
The search and reasoning processes have been optimized, increasing the
modularity of the code. The flexibility of information retrieval and
processing has been improved. The refactored code structure is now
clearer, making it easier to understand and extend the functionality of
the deep research module.

### What problem does this PR solve?

increase  the modularity of the code

### Type of change

- [x] Refactoring

Co-authored-by: wenju.li <wenju.li@deepctr.cn>