test: add tests for isinstance guard on api_base fast path

tests/test_litellm/litellm_core_utils/test_get_api_base.py

@@ -0,0 +1,16 @@
+from litellm.litellm_core_utils.llm_response_utils.get_api_base import get_api_base
+
+
+def test_get_api_base_rejects_non_string_api_base():
+    """When api_base is in dict but not a string, fast path should be skipped."""
+    result = get_api_base(model="gpt-3.5-turbo", optional_params={"api_base": 123})
+    assert result is None or isinstance(result, str)
+
+
+def test_get_api_base_fast_path_returns_string():
+    """When api_base is a valid string in dict, fast path should return it directly."""
+    result = get_api_base(
+        model="gpt-3.5-turbo",
+        optional_params={"api_base": "https://my-proxy.example.com"},
+    )
+    assert result == "https://my-proxy.example.com"