s3: fix remote object not caching (#7790)

* s3: fix remote object not caching

* s3: address review comments for remote object caching

- Fix leading slash in object name by using strings.TrimPrefix
- Return cached entry from CacheRemoteObjectToLocalCluster to get updated local chunk locations
- Reuse existing helper function instead of inline gRPC call

* s3/filer: add singleflight deduplication for remote object caching

- Add singleflight.Group to FilerServer to deduplicate concurrent cache operations
- Wrap CacheRemoteObjectToLocalCluster with singleflight to ensure only one
  caching operation runs per object when multiple clients request the same file
- Add early-return check for already-cached objects
- S3 API calls filer gRPC with timeout and graceful fallback on error
- Clear negative bucket cache when bucket is created via weed shell
- Add integration tests for remote cache with singleflight deduplication

This benefits all clients (S3, HTTP, Hadoop) accessing remote-mounted objects
by preventing redundant cache operations and improving concurrent access performance.

Fixes: https://github.com/seaweedfs/seaweedfs/discussions/7599

* fix: data race in concurrent remote object caching

- Add mutex to protect chunks slice from concurrent append
- Add mutex to protect fetchAndWriteErr from concurrent read/write
- Fix incorrect error check (was checking assignResult.Error instead of parseErr)
- Rename inner variable to avoid shadowing fetchAndWriteErr

* fix: address code review comments

- Remove duplicate remote caching block in GetObjectHandler, keep only singleflight version
- Add mutex protection for concurrent chunk slice and error access (data race fix)
- Use lazy initialization for S3 client in tests to avoid panic during package load
- Fix markdown linting: add language specifier to code fence, blank lines around tables
- Add 'all' target to Makefile as alias for test-with-server
- Remove unused 'util' import

* style: remove emojis from test files

* fix: add defensive checks and sort chunks by offset

- Add nil check and type assertion check for singleflight result
- Sort chunks by offset after concurrent fetching to maintain file order

* fix: improve test diagnostics and path normalization

- runWeedShell now returns error for better test diagnostics
- Add all targets to .PHONY in Makefile (logs-primary, logs-remote, health)
- Strip leading slash from normalizedObject to avoid double slashes in path

---------

Co-authored-by: chrislu <chris.lu@gmail.com>
Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>

.gitignore

@@ -126,3 +126,7 @@ weed-iam
 test/kafka/kafka-client-loadtest/weed-linux-arm64
 /test/tus/filerldb2
 coverage.out
+/test/s3/remote_cache/test-primary-data
+/test/s3/remote_cache/test-remote-data
+test/s3/remote_cache/remote-server.pid
+test/s3/remote_cache/primary-server.pid