fix: skip system messages in OpenRouter cacheAtDepth calculation (#5230)

System messages in the OpenRouter messages array were being counted
toward depth and could receive cache_control breakpoints. Since
OpenRouter hoists system messages into Claude's separate system
parameter, this misplaced breakpoints and could prevent caching
entirely if the hoisted content fell below minimum cache size.

Closes #5227

src/prompt-converters.js

@@ -1026,6 +1026,11 @@ export function cachingAtDepthForOpenRouterClaude(messages, cachingAtDepth, ttl)
 
         passedThePrefill = true;
 
+        // Skip system messages so they don't affect depth counting or receive cache breakpoints
+        if (messages[i].role === 'system') {
+            continue;
+        }
+
         if (messages[i].role !== previousRoleName) {
             if (depth === cachingAtDepth || depth === cachingAtDepth + 2) {
                 const content = messages[i].content;