docs: note Gemini OpenAI-compat wire requirements for tool calls

Adds a compat note to tutorial/09-ai-endpoint covering the tool_calls + name fields that Gemini's v1beta/openai layer requires and atmosphere-ai 4.0.39 serializes.

Author: jfarcand

docs/src/content/docs/tutorial/09-ai-endpoint.md

@@ -340,6 +340,17 @@ Auto-detection resolves the base URL from the model name:
 - All other remote models route to `https://generativelanguage.googleapis.com/v1beta/openai`
 - Local mode routes to `http://localhost:11434/v1` (Ollama)
 
+:::note[Gemini OpenAI-compat wire contract]
+Gemini's `v1beta/openai` layer is stricter than OpenAI itself in two places:
+assistant messages carrying a tool call must emit the full `tool_calls` array
+(not null content), and tool messages must emit the `name` field so Gemini
+can populate its native `function_response.name`. `OpenAiCompatibleClient`
+serializes both unconditionally — OpenAI treats them as additive. If you
+ever see `GenerateContentRequest…function_response.name: Name cannot be
+empty`, you are running an older build; upgrade `atmosphere-ai` to `4.0.39`
+or later.
+:::
+
 ### Atmosphere Init-Params
 
 ```java