fix: 修复gpt->claude转换无法命中codex缓存问题

backend/internal/service/openai_sticky_compat.go

@@ -29,6 +29,13 @@ func openAIStickyCompatStats() (legacyReadFallbackTotal, legacyReadFallbackHit,
 		openAIStickyLegacyDualWriteTotal.Load()
 }
 
+// DeriveSessionHashFromSeed computes the current-format sticky-session hash
+// from an arbitrary seed string.
+func DeriveSessionHashFromSeed(seed string) string {
+	currentHash, _ := deriveOpenAISessionHashes(seed)
+	return currentHash
+}
+
 func deriveOpenAISessionHashes(sessionID string) (currentHash string, legacyHash string) {
 	normalized := strings.TrimSpace(sessionID)
 	if normalized == "" {