Fix Gemini tool calling on 3.6+ by using documented function response role (#24013)

Gemini 3.6 and newer reject role="function" on the function response
Content with 400 INVALID_ARGUMENT, breaking any chat query that triggers
a tool call. The tool call itself succeeds; only the hand-back to the
model fails, and because the error surfaces mid-stream the request still
returns HTTP 200, so it is easy to miss.

Google's function calling documentation specifies role="user" for
returning function results:

    contents.append(response.candidates[0].content)
    contents.append(types.Content(role="user", parts=[function_response_part]))

https://ai.google.dev/gemini-api/docs/generate-content/function-calling

Verified with my local setup.

frigate/genai/plugins/gemini.py

@@ -245,7 +245,7 @@ class GeminiClient(GenAIClient):
                     )
                     gemini_messages.append(
                         types.Content(
-                            role="function",
+                            role="user",
                             parts=[
                                 types.Part.from_function_response(
                                     name=msg.get("name")
@@ -501,7 +501,7 @@ class GeminiClient(GenAIClient):
                     )
                     gemini_messages.append(
                         types.Content(
-                            role="function",
+                            role="user",
                             parts=[
                                 types.Part.from_function_response(
                                     name=msg.get("name")