add unit test coverage for record-storage cluster (Empty / VFS / Sequential)

[aglinxinyuan]

Background

Three modules in engine/common/storage currently lack a dedicated unit-spec:

Source class	Package	Purpose
SequentialRecordStorage	org.apache.texera.amber.engine.common.storage	Abstract sequential-record reader/writer + getStorage factory
VFSRecordStorage	(same)	Apache Commons VFS concrete implementation
EmptyRecordStorage	(same)	Null-object implementation (no-op writer / EOF reader / always-false containsFolder)

All three are reachable from production code (SequentialRecordStorage.getStorage is the factory used by checkpoint logging) but none have characterization tests. A regression in any of these would only surface as a downstream serde / replay failure.

What we want pinned

Behavior we want to lock in:

Area	Contract
SequentialRecordStorage.getStorage(None)	returns an EmptyRecordStorage
SequentialRecordStorage.getStorage(Some(file://…))	returns a VFSRecordStorage
SequentialRecordStorage.getStorage(Some(hdfs://…))	dispatches to HDFSRecordStorage (path covered without actually opening an HDFS connection by asserting the constructor blows up on a non-resolvable host rather than silently returning VFSRecordStorage)
SequentialRecordWriter / SequentialRecordReader	round-trip a sequence of records through AmberRuntime.serde (size-prefixed framing)
SequentialRecordStorage.fetchAllRecords	iterates all records returned by the underlying reader
VFSRecordStorage constructor	auto-creates the target folder when it does not exist
VFSRecordStorage.getWriter / getReader	round-trip a record through a local file:// URI
VFSRecordStorage.deleteStorage	removes the on-disk folder created by the constructor
VFSRecordStorage.containsFolder	distinguishes existing folder vs. existing file vs. missing entry
EmptyRecordStorage.getWriter	returns a writer backed by NullOutputStream (writes are silently discarded)
EmptyRecordStorage.getReader	returns a reader that yields zero records
EmptyRecordStorage.deleteStorage / containsFolder	are no-op and always-false respectively

Scope

• New spec files (one per source class per the spec-filename convention):
  • SequentialRecordStorageSpec.scala
  • VFSRecordStorageSpec.scala
  • EmptyRecordStorageSpec.scala
• No production-code changes.
• Tests use the production wire path (AmberRuntime.serde) the same way CheckpointSubsystemSpec / ClientEventSpec do (a suite-local ActorSystem injected into AmberRuntime via reflection, torn down in afterAll).