fix cohere rerank base_url default (#11353)

### What problem does this PR solve?

**Cohere rerank base_url default handling**

- Background: When no rerank base URL is configured, the settings
pipeline was passing an empty string through RERANK_CFG →
TenantLLMService → CoHereRerank, so the Cohere client received
base_url="" and produced “missing protocol” errors during rerank calls.

- What changed: The CoHereRerank constructor now only forwards base_url
to the Cohere client when it isn’t empty/whitespace, causing the client
to fall back to its default API endpoint otherwise.

- Why it matters: This prevents invalid URL construction in the rerank
workflow and keeps tests/sanity checks that rely on the default Cohere
endpoint from failing when no custom base URL is specified.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: Philipp Heyken Soares <philipp.heyken-soares@am.ai>

rag/llm/rerank_model.py

@@ -234,7 +234,11 @@ class CoHereRerank(Base):
     def __init__(self, key, model_name, base_url=None):
         from cohere import Client
 
-        self.client = Client(api_key=key, base_url=base_url)
+        # Only pass base_url if it's a non-empty string, otherwise use default Cohere API endpoint
+        client_kwargs = {"api_key": key}
+        if base_url and base_url.strip():
+            client_kwargs["base_url"] = base_url
+        self.client = Client(**client_kwargs)
         self.model_name = model_name.split("___")[0]
 
     def similarity(self, query: str, texts: list):