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");

rust/crates/api/tests/client_integration.rs

@@ -4,9 +4,10 @@ use std::sync::{Mutex as StdMutex, OnceLock};
 use std::time::Duration;
 
 use api::{
-    ApiClient, ApiError, AuthSource, ContentBlockDelta, ContentBlockDeltaEvent,
+    AnthropicClient, ApiClient, ApiError, AuthSource, ContentBlockDelta, ContentBlockDeltaEvent,
     ContentBlockStartEvent, InputContentBlock, InputMessage, MessageDeltaEvent, MessageRequest,
-    OutputContentBlock, ProviderClient, StreamEvent, ToolChoice, ToolDefinition,
+    OutputContentBlock, PromptCache, PromptCacheConfig, ProviderClient, StreamEvent, ToolChoice,
+    ToolDefinition,
 };
 use serde_json::json;
 use telemetry::{ClientIdentity, MemoryTelemetrySink, SessionTracer, TelemetryEvent};
@@ -562,10 +563,10 @@ async fn send_message_tracks_unexpected_prompt_cache_breaks() {
     let request = sample_request(false);
     let client = AnthropicClient::new("test-key")
         .with_base_url(server.base_url())
-        .with_prompt_cache(PromptCache::with_config(api::PromptCacheConfig {
+        .with_prompt_cache(PromptCache::with_config(PromptCacheConfig {
             session_id: "break-session".to_string(),
             completion_ttl: Duration::from_secs(0),
-            ..api::PromptCacheConfig::default()
+            ..PromptCacheConfig::default()
         }));
 
     client