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

backend/internal/service/openai_compat_prompt_cache_key.go

@@ -0,0 +1,81 @@
+package service
+
+import (
+	"encoding/json"
+	"strings"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/apicompat"
+)
+
+const compatPromptCacheKeyPrefix = "compat_cc_"
+
+func shouldAutoInjectPromptCacheKeyForCompat(model string) bool {
+	switch normalizeCodexModel(strings.TrimSpace(model)) {
+	case "gpt-5.4", "gpt-5.3-codex":
+		return true
+	default:
+		return false
+	}
+}
+
+func deriveCompatPromptCacheKey(req *apicompat.ChatCompletionsRequest, mappedModel string) string {
+	if req == nil {
+		return ""
+	}
+
+	normalizedModel := normalizeCodexModel(strings.TrimSpace(mappedModel))
+	if normalizedModel == "" {
+		normalizedModel = normalizeCodexModel(strings.TrimSpace(req.Model))
+	}
+	if normalizedModel == "" {
+		normalizedModel = strings.TrimSpace(req.Model)
+	}
+
+	seedParts := []string{"model=" + normalizedModel}
+	if req.ReasoningEffort != "" {
+		seedParts = append(seedParts, "reasoning_effort="+strings.TrimSpace(req.ReasoningEffort))
+	}
+	if len(req.ToolChoice) > 0 {
+		seedParts = append(seedParts, "tool_choice="+normalizeCompatSeedJSON(req.ToolChoice))
+	}
+	if len(req.Tools) > 0 {
+		if raw, err := json.Marshal(req.Tools); err == nil {
+			seedParts = append(seedParts, "tools="+normalizeCompatSeedJSON(raw))
+		}
+	}
+	if len(req.Functions) > 0 {
+		if raw, err := json.Marshal(req.Functions); err == nil {
+			seedParts = append(seedParts, "functions="+normalizeCompatSeedJSON(raw))
+		}
+	}
+
+	firstUserCaptured := false
+	for _, msg := range req.Messages {
+		switch strings.TrimSpace(msg.Role) {
+		case "system":
+			seedParts = append(seedParts, "system="+normalizeCompatSeedJSON(msg.Content))
+		case "user":
+			if !firstUserCaptured {
+				seedParts = append(seedParts, "first_user="+normalizeCompatSeedJSON(msg.Content))
+				firstUserCaptured = true
+			}
+		}
+	}
+
+	return compatPromptCacheKeyPrefix + hashSensitiveValueForLog(strings.Join(seedParts, "|"))
+}
+
+func normalizeCompatSeedJSON(v json.RawMessage) string {
+	if len(v) == 0 {
+		return ""
+	}
+	var tmp any
+	if err := json.Unmarshal(v, &tmp); err != nil {
+		return string(v)
+	}
+	out, err := json.Marshal(tmp)
+	if err != nil {
+		return string(v)
+	}
+	return string(out)
+}