fix: add defensive None checks for model parameter

This fixes TypeError: argument of type 'NoneType' is not iterable
that occurs when model=None is passed to get_llm_provider and related
functions.

Changes:
- Add early validation in get_llm_provider to raise clear error for None model
- Add defensive None checks in handle_cohere_chat_model_custom_llm_provider
- Add defensive None checks in handle_anthropic_text_model_custom_llm_provider
- Fix metadata None handling in utils.py for aresponses call type
- Add comprehensive unit tests for None model handling

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_litellm/litellm_core_utils/test_get_llm_provider_logic.py

@@ -0,0 +1,76 @@
+"""
+Tests for get_llm_provider_logic.py
+
+Focuses on None model handling to prevent 'argument of type NoneType is not iterable' errors.
+"""
+
+import pytest
+
+import litellm
+from litellm.litellm_core_utils.get_llm_provider_logic import (
+    get_llm_provider,
+    handle_anthropic_text_model_custom_llm_provider,
+    handle_cohere_chat_model_custom_llm_provider,
+)
+
+
+class TestHandleNoneModel:
+    """Tests for handling None model parameter"""
+
+    def test_handle_cohere_chat_model_with_none_model(self):
+        """Test that handle_cohere_chat_model_custom_llm_provider handles None model gracefully"""
+        # Should not raise TypeError: argument of type 'NoneType' is not iterable
+        result = handle_cohere_chat_model_custom_llm_provider(
+            model=None, custom_llm_provider="cohere"
+        )
+        assert result == (None, "cohere")
+
+    def test_handle_anthropic_text_model_with_none_model(self):
+        """Test that handle_anthropic_text_model_custom_llm_provider handles None model gracefully"""
+        # Should not raise TypeError: argument of type 'NoneType' is not iterable
+        result = handle_anthropic_text_model_custom_llm_provider(
+            model=None, custom_llm_provider="anthropic"
+        )
+        assert result == (None, "anthropic")
+
+    def test_get_llm_provider_with_none_model_raises_clear_error(self):
+        """Test that get_llm_provider raises a clear error when model is None"""
+        # The ValueError is caught by the function's exception handler and wrapped in BadRequestError
+        with pytest.raises(litellm.exceptions.BadRequestError) as exc_info:
+            get_llm_provider(model=None)
+
+        assert "model parameter is required but was None" in str(exc_info.value)
+
+
+class TestValidModelHandling:
+    """Tests to ensure valid models still work correctly after the None checks"""
+
+    def test_handle_cohere_chat_model_with_valid_model(self):
+        """Test that valid cohere models still work"""
+        result = handle_cohere_chat_model_custom_llm_provider(
+            model="cohere/command-r-plus", custom_llm_provider=None
+        )
+        # Should parse the model correctly
+        assert result[0] == "command-r-plus" or result[0] == "cohere/command-r-plus"
+
+    def test_handle_cohere_chat_model_with_model_without_slash(self):
+        """Test that models without slash work"""
+        result = handle_cohere_chat_model_custom_llm_provider(
+            model="command-r-plus", custom_llm_provider="cohere"
+        )
+        assert result[0] == "command-r-plus"
+
+    def test_handle_anthropic_text_model_with_valid_model(self):
+        """Test that valid anthropic models still work"""
+        result = handle_anthropic_text_model_custom_llm_provider(
+            model="anthropic/claude-2", custom_llm_provider=None
+        )
+        # Should parse the model correctly
+        assert "claude-2" in result[0]
+
+    def test_handle_anthropic_text_model_with_model_without_slash(self):
+        """Test that models without slash work"""
+        result = handle_anthropic_text_model_custom_llm_provider(
+            model="claude-2", custom_llm_provider="anthropic"
+        )
+        assert result[0] == "claude-2"