fix(openai): strip unsupported passthrough fields

backend/internal/service/openai_passthrough_normalization_test.go

@@ -0,0 +1,33 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/tidwall/gjson"
+)
+
+func TestNormalizeOpenAIPassthroughOAuthBody_RemovesUnsupportedUser(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","input":"hello","user":"user_123","metadata":{"user_id":"user_123"},"prompt_cache_retention":"24h","safety_identifier":"sid","stream_options":{"include_usage":true}}`)
+
+	normalized, changed, err := normalizeOpenAIPassthroughOAuthBody(body, false)
+	require.NoError(t, err)
+	require.True(t, changed)
+	for _, field := range openAIChatGPTInternalUnsupportedFields {
+		require.False(t, gjson.GetBytes(normalized, field).Exists(), "%s should be stripped", field)
+	}
+	require.True(t, gjson.GetBytes(normalized, "stream").Bool())
+	require.False(t, gjson.GetBytes(normalized, "store").Bool())
+}
+
+func TestNormalizeOpenAIPassthroughOAuthBody_CompactRemovesUnsupportedUser(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","input":"hello","user":"user_123","metadata":{"user_id":"user_123"},"stream":true,"store":true}`)
+
+	normalized, changed, err := normalizeOpenAIPassthroughOAuthBody(body, true)
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.False(t, gjson.GetBytes(normalized, "user").Exists())
+	require.False(t, gjson.GetBytes(normalized, "metadata").Exists())
+	require.False(t, gjson.GetBytes(normalized, "stream").Exists())
+	require.False(t, gjson.GetBytes(normalized, "store").Exists())
+}