fix: S3 remote storage cold-cache read fails with 'size reported but no content available' (#7815)
When a remote-only entry's initial caching attempt times out or fails,
streamFromVolumeServers() now detects this case and retries caching
synchronously before streaming, similar to how the filer server handles
remote-only entries.
Changes:
- Modified streamFromVolumeServers() to check entry.IsInRemoteOnly()
before treating missing chunks as a data integrity error
- Added doCacheRemoteObject() as the core caching function (calls filer gRPC)
- Added buildRemoteObjectPath() helper to reduce code duplication
- Refactored cacheRemoteObjectWithDedup() and cacheRemoteObjectForStreaming()
to reuse the shared functions
- Added integration tests for remote storage scenarios
Fixes https://github.com/seaweedfs/seaweedfs/issues/7815
