fix: cover merged prompt cache behavior

rust/crates/api/src/prompt_cache.rs

@@ -500,15 +500,22 @@ fn stable_hash_bytes(bytes: &[u8]) -> u64 {
 
 #[cfg(test)]
 mod tests {
+    use std::sync::{Mutex, OnceLock};
     use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
     use super::{
         detect_cache_break, read_json, request_hash_hex, sanitize_path_segment, PromptCache,
         PromptCacheConfig, PromptCachePaths, TrackedPromptState, REQUEST_FINGERPRINT_PREFIX,
     };
-    use crate::test_env_lock;
     use crate::types::{InputMessage, MessageRequest, MessageResponse, OutputContentBlock, Usage};
 
+    fn test_env_lock() -> std::sync::MutexGuard<'static, ()> {
+        static LOCK: OnceLock<Mutex<()>> = OnceLock::new();
+        LOCK.get_or_init(|| Mutex::new(()))
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner)
+    }
+
     #[test]
     fn path_builder_sanitizes_session_identifier() {
         let paths = PromptCachePaths::for_session("session:/with spaces");