Add Cassandra 6.0 support - #236

Add Cassandra 6.0 support

Cassandra Analytics reads, bulk writes and consumes CDC through one bridge jar per major version. This adds the SIXZERO bridge, pinned to 6.0-alpha2 from Maven Central, in five new modules that copy and override the five-zero and four-zero sources.

Five 6.0 changes needed work:

 - SSTable versions big-pa and bti-ea, both added for compression dictionary metadata in the CompressionInfo component.
 - A trailing dictionary section in CompressionInfo. The six-zero CompressionMetadata deserializes it, interns one dictionary per dictId so that readers of one SSTable share the zstd compressor that ZstdDictionaryCompressor keys by dictId, and releases the references it holds through close(). SSTableCache gained a removal listener to call close(), and SSTableReader holds a second reference for the life of a stream.
 - CEP-21 Transactional Cluster Metadata retypes Schema.instance to SchemaProvider and replaces transform() with submit(), which notifies no listener offline, so SchemaUpdater initializes the keyspace instances by hand after ClusterMetadataService.initializeForClients().
 - Commit log version 9 and messaging version 14, read through the shaded 6.0 deserializers.
 - PartitionUpdate.PartitionUpdateSerializer.deserialize catches UnknownTableException and rethrows CoordinatorBehindException(message, exception), a plain RuntimeException. BufferingCommitLogReader skips a mutation whose table is absent from Schema.instance, because one commit log serves every table and a CDC reader is given only the tables it was supplied, and that skip never happened on 6.0: catch (UnknownTableException) did not fire, so the failure reached handleUnrecoverableError, which calls requestTermination(), and the scanner re-read the same segment for as long as it existed. Measured against 6.0-alpha2 with one table missing from the supplied schema, the reader re-read one segment for eight minutes and published nothing while cdc_raw grew from 76 segments to 125. The reader now looks through the cause chain rather than matching the declared type. Skipping is safe wherever the wrapping happens, because readSection has already read the mutation's bytes out of the file before calling this method, so the file pointer is past the mutation either way.

Four classes the six-zero modules first copied are gone again, because the module below serves every version. The four-zero determineSupportedPartitioner takes Murmur3Partitioner.instance and RandomPartitioner.instance rather than a new instance, since 6.0 makes both constructors non-public, which leaves the six-zero SSTableWriterImplementation with nothing to change. writeTombstoneSSTable builds a CQLSSTableWriter, which on 6.0 accepts any modification statement and writes the range tombstones of a slice delete through the same DeleteStatement.createSlices path the copy used, so the 6.0 bridge needs no SSTableTombstoneWriter. AbstractCqlList.addCell built the cell path of an expiring list cell from UUIDGen.getTimeUUIDBytes(), which 5.0 removed, so both branches now call randomCellPath(), the per-version hook the other branch already used; that path is reached only from CdcBridge.log, a test utility, so no released bridge wrote a list cell through it. Five-zero's TimeUUID overrides randomValue, which left the six-zero AbstractTimeUUID override unreachable.

Two defects in the existing bridges surfaced while testing the copies and are fixed here. CassandraSchema.unregisterNonCdcTable called SchemaUpdater.load, which on 5.0 only adds a keyspace and throws AlreadyExistsException for one that exists, so the table stayed registered; removeTables replaces the keyspace metadata instead. On 5.0 that commit reaches Keyspace.dropCf, which needs machinery a client-mode process never started, so removeTables clears the initialized flag for the commit only.

BridgeVersionConsistencyTest asserts that every type a bridge hands out reports the bridge's own version, which catches a six-zero module that silently inherits a version-bearing class from five-zero through the Gradle copy-forward.

 patch by Mick Semb Wever; reviewed by Saranya Krishnakumar for CASSANALYTICS-37

Assisted-by: Claude Code:claude-opus-5