fix(mcp): Fix Gemini conversation format issue with MCP auto-execution (#16592)

When using MCP tools with require_approval='never' and Gemini models,
the follow-up call after tool execution was failing with:

  'Please ensure that function call turn comes immediately after a user
   turn or after a function response turn.'

This was caused by adding an empty assistant message between the user
message and function calls, which violates Gemini's conversation format
requirements.

Changes:
- Only add assistant message to follow-up input if it contains actual content
- Allow function calls to come directly after user messages (as Gemini requires)
- Add explanatory comments about Gemini's format requirements

This fix allows MCP auto-execution to work correctly with Gemini models
while maintaining compatibility with other models.

Fixes: #[issue-number-if-any]

litellm/responses/mcp/litellm_proxy_mcp_handler.py

@@ -526,8 +526,10 @@ class LiteLLM_Proxy_MCP_Handler:
                     else:
                         assistant_message_content.append(content)
 
-        # Add assistant message with content and function calls
-        if assistant_message_content or function_calls:
+        # Add assistant message only if there's actual content (not empty)
+        # For example, gemini requires that function call turns come immediately after user turns,
+        # so we should not add empty assistant messages
+        if assistant_message_content:
             follow_up_input.append(
                 {
                     "type": "message",
@@ -536,9 +538,9 @@ class LiteLLM_Proxy_MCP_Handler:
                 }
             )
 
-            # Add function calls after assistant message
-            for function_call in function_calls:
-                follow_up_input.append(function_call)
+        # Add function calls (these can come directly after user message for LLM)
+        for function_call in function_calls:
+            follow_up_input.append(function_call)
 
         # Add tool results (function call outputs)
         for tool_result in tool_results: