[flink] Support stream read Chain Table

[yunfengzhou-hub]

Purpose

Chain Table (chain-table.enabled=true) separates data into a snapshot branch (batch-imported full partitions) and a delta branch (incremental updates). Prior to this change, streaming read was not supported because the standard DataTableStreamScan is unaware of the two-branch architecture.

This PR introduces ChainTableFileStoreTable (a wrapper over FallbackReadFileStoreTable) and ChainTableStreamScan which implements a two-phase streaming scan: Phase 1 does a full load by reading delta data pinned to the current snapshot and merging snapshot files for overlapping partitions; Phase 2 incrementally monitors the delta branch only, returning DataSplit(isStreaming=true) for changelog passthrough. The snapshot-pinning strategy makes the Phase 1 / Phase 2 boundary deterministic — no overlap or data loss regardless of concurrent commits.

Tests

Added FlinkChainTableITCase with 16 tests (all passing, ~75s):

• Full load with snapshot+delta overlap, empty delta, empty snapshot
• Changelog passthrough (-U/+U) with changelog-producer=input
• Snapshot OVERWRITE does not trigger streaming output
• Stateless restart and stateful restart (MiniCluster checkpoint/restore)
• WHERE predicate forwarding, withShard forwarding
• scan.mode=latest bypass, changelog-producer=none rejection
• restore(id, scanAll=true) and restore(null, scanAll=true) state reset
• chain-partition-keys group partition streaming