### What problem does this PR solve?
default OpenAI chat completions to non-stream when `stream` is omitted
https://github.com/infiniflow/ragflow/issues/15356
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Python implementation of the Go-based model_provider API suite.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
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Co-authored-by: bill <yibie_jingnian@163.com>
### What problem does this PR solve?
Fix: /openai/<chat_id>/chat/completions not aware of session_id
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
extend restful api suite
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
- [x] Other (please describe): test
### What problem does this PR solve?
add new testing suite for the new restful api endpoints meant to replace
http and web api tests
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
- [x] Other (please describe): test
