fix(openai): inject stable compat prompt_cache_key for codex oauth chat-completions path

backend/internal/service/openai_compat_prompt_cache_key_test.go

@@ -0,0 +1,64 @@
+package service
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/apicompat"
+	"github.com/stretchr/testify/require"
+)
+
+func mustRawJSON(t *testing.T, s string) json.RawMessage {
+	t.Helper()
+	return json.RawMessage(s)
+}
+
+func TestShouldAutoInjectPromptCacheKeyForCompat(t *testing.T) {
+	require.True(t, shouldAutoInjectPromptCacheKeyForCompat("gpt-5.4"))
+	require.True(t, shouldAutoInjectPromptCacheKeyForCompat("gpt-5.3"))
+	require.True(t, shouldAutoInjectPromptCacheKeyForCompat("gpt-5.3-codex"))
+	require.False(t, shouldAutoInjectPromptCacheKeyForCompat("gpt-4o"))
+}
+
+func TestDeriveCompatPromptCacheKey_StableAcrossLaterTurns(t *testing.T) {
+	base := &apicompat.ChatCompletionsRequest{
+		Model: "gpt-5.4",
+		Messages: []apicompat.ChatMessage{
+			{Role: "system", Content: mustRawJSON(t, `"You are helpful."`)},
+			{Role: "user", Content: mustRawJSON(t, `"Hello"`)},
+		},
+	}
+	extended := &apicompat.ChatCompletionsRequest{
+		Model: "gpt-5.4",
+		Messages: []apicompat.ChatMessage{
+			{Role: "system", Content: mustRawJSON(t, `"You are helpful."`)},
+			{Role: "user", Content: mustRawJSON(t, `"Hello"`)},
+			{Role: "assistant", Content: mustRawJSON(t, `"Hi there!"`)},
+			{Role: "user", Content: mustRawJSON(t, `"How are you?"`)},
+		},
+	}
+
+	k1 := deriveCompatPromptCacheKey(base, "gpt-5.4")
+	k2 := deriveCompatPromptCacheKey(extended, "gpt-5.4")
+	require.Equal(t, k1, k2, "cache key should be stable across later turns")
+	require.NotEmpty(t, k1)
+}
+
+func TestDeriveCompatPromptCacheKey_DiffersAcrossSessions(t *testing.T) {
+	req1 := &apicompat.ChatCompletionsRequest{
+		Model: "gpt-5.4",
+		Messages: []apicompat.ChatMessage{
+			{Role: "user", Content: mustRawJSON(t, `"Question A"`)},
+		},
+	}
+	req2 := &apicompat.ChatCompletionsRequest{
+		Model: "gpt-5.4",
+		Messages: []apicompat.ChatMessage{
+			{Role: "user", Content: mustRawJSON(t, `"Question B"`)},
+		},
+	}
+
+	k1 := deriveCompatPromptCacheKey(req1, "gpt-5.4")
+	k2 := deriveCompatPromptCacheKey(req2, "gpt-5.4")
+	require.NotEqual(t, k1, k2, "different first user messages should yield different keys")
+}