fix: Cohere - better handling of COHERE_API_URL (#6407)

* extract API URL from lazy import

* improve solution

haystack/preview/components/generators/cohere.py

@@ -33,7 +33,7 @@ def __init__(
         api_key: Optional[str] = None,
         model_name: str = "command",
         streaming_callback: Optional[Callable] = None,
-        api_base_url: str = COHERE_API_URL,
+        api_base_url: Optional[str] = None,
         **kwargs,
     ):
         """
@@ -61,13 +61,18 @@ def __init__(
                           The format is {token_id: bias} where bias is a float between -10 and 10.
 
         """
+        cohere_import.check()
+
         if not api_key:
             api_key = os.environ.get("COHERE_API_KEY")
         if not api_key:
             raise ValueError(
                 "CohereGenerator needs an API key to run. Either provide it as init parameter or set the env var COHERE_API_KEY."
             )
 
+        if not api_base_url:
+            api_base_url = COHERE_API_URL
+
         self.api_key = api_key
         self.model_name = model_name
         self.streaming_callback = streaming_callback