### What problem does this PR solve?
The Go model-driver layer () has ~38,700 lines across 109 files. Roughly
74% of that is boilerplate duplicated into every driver: identical HTTP
client setup, the same 65-line SSE scanner loop, and 10-11 one-line "not
supported" stub methods per driver. Any fix must be manually propagated
to every file. Closes#15820.
This PR establishes the three shared utility files that form the
foundation for incremental driver migration:
---
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
- [x] Refactoring
---------
Co-authored-by: Haruko386 <tryeverypossible@163.com>
### What problem does this PR solve?
As title
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Refactoring
- Adds an `Astraflow` Go driver so the new API server can route
Astraflow (UCloud ModelVerse) chat instances, matching the existing
Python `AstraflowChat` (`rag/llm/chat_model.py:1237`). Follows the same
SaaS-driver shape used for Avian, Novita, TogetherAI, Replicate,
DeepInfra, Upstage, and LongCat.
Closes#15062
---------
Co-authored-by: Jin Hai <haijin.chn@gmail.com>