fix: align LocalAI vision constructor arguments (#16920)

## What problem does this PR solve?

`TenantLLMService.model_instance` constructs vision providers with
`lang` as the third positional argument and `base_url` as a keyword
argument.

`LocalAICV` declared `base_url` as its third parameter, causing:

```text
TypeError: LocalAICV.__init__() got multiple values for argument 'base_url'
```

This prevents LocalAI vision models from being used during document
parsing.

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

rag/llm/cv_model.py

@@ -728,7 +728,7 @@ class OpenRouterCV(GptV4):
 class LocalAICV(GptV4):
     _FACTORY_NAME = "LocalAI"
 
-    def __init__(self, key, model_name, base_url, lang="Chinese", **kwargs):
+    def __init__(self, key, model_name, lang="Chinese", base_url="", **kwargs):
         if not base_url:
             raise ValueError("Local cv model url cannot be None")
         base_url = ensure_v1(base_url)