fix: Support batch requests with comma-separated models in validate_model_access (#18909)

This fixes a breaking change where batch completion requests with comma-separated
model strings (e.g., 'gpt-3.5-turbo,fake-openai-endpoint') were failing validation.

The validate_model_access function now:
- Detects comma-separated model strings
- Validates each model individually
- Provides clear error messages for inaccessible models in batch requests
- Maintains backward compatibility for single model validation

Fixes test_batch_chat_completions test failure.

litellm/proxy/utils.py

@@ -4479,21 +4479,35 @@ def validate_model_access(
 ) -> None:
     """
     Validate that a model is accessible to the user.
+    Supports batch requests with comma-separated model IDs.
 
     Args:
-        model_id: The model ID to validate
+        model_id: The model ID to validate (can be comma-separated for batch requests)
         available_models: List of models available to the user
 
     Raises:
         HTTPException: If the model is not accessible
     """
-    if model_id not in available_models:
-        raise HTTPException(
-            status_code=404,
-            detail="The model `{}` does not exist or is not accessible".format(
-                model_id
-            ),
-        )
+    # Handle batch requests with comma-separated models
+    if "," in model_id:
+        models = [m.strip() for m in model_id.split(",")]
+        inaccessible_models = [m for m in models if m not in available_models]
+        if inaccessible_models:
+            raise HTTPException(
+                status_code=404,
+                detail="The following model(s) do not exist or are not accessible: {}".format(
+                    ", ".join(inaccessible_models)
+                ),
+            )
+    else:
+        # Single model validation
+        if model_id not in available_models:
+            raise HTTPException(
+                status_code=404,
+                detail="The model `{}` does not exist or is not accessible".format(
+                    model_id
+                ),
+            )
 
 
 def _path_matches_pattern(path: str, pattern: str) -> bool: