Fix critical JSON field collision bug in /_nodes/stats/file_cache API

Problem: PINNED field data overwrites USED field data due to using same JSON key
Solution: Use PINNED_IN_BYTES constant for pinned data instead of USED_IN_BYTES

This eliminates JSON field collision and ensures correct cache usage reporting in API responses.

Signed-off-by: tanishq ranjan <[email protected]>

Author: tqranjan

server/src/main/java/org/opensearch/index/store/remote/filecache/FileCacheStats.java

@@ -145,6 +145,7 @@ static final class Fields {
         static final String USED = "used";
         static final String PINNED = "pinned";
         static final String USED_IN_BYTES = "used_in_bytes";
+        static final String PINNED_IN_BYTES = "pinned_in_bytes";
         static final String EVICTIONS = "evictions";
         static final String EVICTIONS_IN_BYTES = "evictions_in_bytes";
         static final String REMOVED = "removed";
@@ -159,7 +160,7 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         builder.startObject(statsType.toString());
         builder.humanReadableField(FileCacheStats.Fields.ACTIVE_IN_BYTES, FileCacheStats.Fields.ACTIVE, new ByteSizeValue(getActive()));
         builder.humanReadableField(FileCacheStats.Fields.USED_IN_BYTES, FileCacheStats.Fields.USED, new ByteSizeValue(getUsed()));
-        builder.humanReadableField(FileCacheStats.Fields.USED_IN_BYTES, Fields.PINNED, new ByteSizeValue(getPinnedUsage()));
+        builder.humanReadableField(FileCacheStats.Fields.PINNED_IN_BYTES, Fields.PINNED, new ByteSizeValue(getPinnedUsage()));
         builder.humanReadableField(
             FileCacheStats.Fields.EVICTIONS_IN_BYTES,
             FileCacheStats.Fields.EVICTIONS,