### What problem does this PR solve?
Implement the vLLM model provider for RAGFlow to fully support local and
self-hosted open-source models (e.g., Qwen, GLM, Llama) via the vLLM
framework, and fix several critical bugs related to model instance
management and API requests.
**Key changes and fixes:**
1. **Added Standard vLLM Provider (`vllm.go`, `vllm.json`):**
- Implemented `VllmModel` driver strictly adhering to the OpenAI API
specification.
- Removed hardcoded and dangerous routing logic (e.g., forcing
`AsyncChat` for Qwen/GLM prefixes), ensuring standard
`/v1/chat/completions` compatibility.
- Refactored `ListModels` to use safe JSON parsing (resolving nil
pointer panics) and standard `GET` requests without bodies.
- Added `APIConfig.Region` fallback logic to prevent empty `base_url`
fetching when checking models.
2. **Fixed `ChatToModelStreamWithSender` Bug (`model_service.go`):**
- Resolved the `model is disabled` error when streaming chat with local
database-saved models.
- Added the missing `if modelInfo.Status == "active"` block to correctly
invoke `NewInstance` and inject the dynamic `base_url` into the provider
driver before starting the SSE stream.
3. **Fixed `ListSupportedModels` Bug (`model_service.go`):**
- Added dynamic `NewInstance` injection for `base_url`. Previously, the
list models function used the static JSON config without injecting
user-configured dynamic URLs from the database, resulting in an
`unsupported protocol scheme ""` error.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
1. support command:
```
RAGFlow(user)> create provider 'vllm' instance 'test' key 'test-key' url 'base-url' region 'abc';
SUCCESS
RAGFlow(user)> list instances from 'vllm';
+----------+----------------------------------------+----------------------------------+--------------+----------------------------------+--------+
| apiKey | extra | id | instanceName | providerID | status |
+----------+----------------------------------------+----------------------------------+--------------+----------------------------------+--------+
| test-key | {"base_url":"base-url","region":"abc"} | 40213c89430311f1a7cf38a74640adcc | test | b4d40e6142d311f1a4f938a74640adcc | enable |
+----------+----------------------------------------+----------------------------------+--------------+----------------------------------+--------+
```
2. support add vllm model
```
RAGFlow(user)> add model 'Qwen/Qwen2-0.5B' to provider 'vllm' instance 'test' with tokens 131072 chat;
SUCCESS
```
3. add vllm chat
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
- [x] Refactoring
---------
Signed-off-by: Jin Hai <haijin.chn@gmail.com>
