fix: use Responses-compatible function tool_choice format

backend/internal/pkg/apicompat/chatcompletions_responses_test.go

@@ -281,6 +281,8 @@ func TestChatCompletionsToResponses_LegacyFunctions(t *testing.T) {
 	var tc map[string]any
 	require.NoError(t, json.Unmarshal(resp.ToolChoice, &tc))
 	assert.Equal(t, "function", tc["type"])
+	assert.Equal(t, "get_weather", tc["name"])
+	assert.NotContains(t, tc, "function")
 }
 
 func TestChatCompletionsToResponses_ServiceTier(t *testing.T) {