fix: return the helpful error for a bare provider name in get_llm_provider

Passing a bare provider name (e.g. get_llm_provider('mistral')) hit the elif
that assumes a '/' and did model.split('/', 1)[1], raising IndexError that got
wrapped into 'GetLLMProvider Exception - list index out of range'. The sibling
if above it already guards this with len(model.split('/')) > 1 (added for issue
1351); the elif was missing the same guard. Add it so a bare provider name
falls through to the intended 'LLM Provider NOT provided' guidance.

litellm/litellm_core_utils/get_llm_provider_logic.py

@@ -218,7 +218,7 @@ def get_llm_provider(
                 dynamic_api_key=dynamic_api_key,
                 litellm_params=litellm_params,
             )
-        elif model.split("/", 1)[0] in litellm.provider_list:
+        elif model.split("/", 1)[0] in litellm.provider_list and len(model.split("/")) > 1:
             custom_llm_provider = model.split("/", 1)[0]
             model = model.split("/", 1)[1]
             if api_base is not None and not isinstance(api_base, str):

tests/test_litellm/litellm_core_utils/test_get_llm_provider_logic.py

@@ -0,0 +1,12 @@
+import pytest
+
+import litellm
+
+
+@pytest.mark.parametrize("model", ["mistral", "openai", "anthropic", "groq", "deepseek"])
+def test_get_llm_provider_bare_provider_name_raises_helpful_error(model):
+    with pytest.raises(litellm.BadRequestError) as exc_info:
+        litellm.get_llm_provider(model=model)
+    message = str(exc_info.value)
+    assert "LLM Provider NOT provided" in message
+    assert "list index out of range" not in message