Restore default shard_path_type to FIXED for snapshot repositories (#20940)

PR #18163 changed the default shard_path_type from FIXED to HASHED_PREFIX,
which causes snapshot blobs to be uploaded outside the configured base_path.
This is a breaking change for users upgrading from 2.x to 3.x, as it breaks
incremental backups and places files outside the expected repository path.

Restoring the default to FIXED to match the 2.x behavior and the existing
IndexId.DEFAULT_SHARD_PATH_TYPE constant.

Signed-off-by: mango766 <mango766@users.noreply.github.com>
Co-authored-by: easonysliu <easonysliu@tencent.com>

Author: mango766

CHANGELOG.md

@@ -66,6 +66,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix stale segment cleanup logic for remote store ([#20976](https://github.com/opensearch-project/OpenSearch/pull/20976))
 - Ensure that transient ThreadContext headers with propagators survive restore ([#169373](https://github.com/opensearch-project/OpenSearch/pull/20854))
 - Handle dependencies between analyzers ([#19248](https://github.com/opensearch-project/OpenSearch/pull/19248))
+- Restore default `shard_path_type` to FIXED for snapshot repositories ([#20643](https://github.com/opensearch-project/OpenSearch/issues/20643))
 - Fix `_field_caps` returning empty results and corrupted field names for `disable_objects: true` mappings ([#20800](https://github.com/opensearch-project/OpenSearch/pull/20800))
 
 ### Dependencies

server/src/main/java/org/opensearch/repositories/blobstore/BlobStoreRepository.java

@@ -418,7 +418,7 @@ protected static long calculateMaxWithinIntLimit(long defaultThresholdOfHeap, lo
 
     public static final Setting<PathType> SHARD_PATH_TYPE = new Setting<>(
         "shard_path_type",
-        PathType.HASHED_PREFIX.toString(),
+        PathType.FIXED.toString(),
         PathType::parseString
     );