[FLINK-40517][checkpointing] Aggregate cross-channel watermarks on the recovery unspilling thread for fan-in rescale

[1996fanrui]

What is the purpose of the change

On the normal recovery path, when a fan-in rescale merges several old input channels into one new channel,
DemultiplexingRecordDeserializer aggregates the recovered non-record elements across the merged channels:
it emits the min watermark (suppressed until every merged channel has one) and ACTIVE if any channel is
active. With checkpointing during recovery enabled that aggregation moves to the unspilling thread, but
ChannelStateFilteringHandler passed watermarks and statuses through verbatim — so a high watermark from one
merged channel advanced the operator watermark prematurely and legitimate in-flight lower-timestamp records
were dropped as late (silent data loss), and an IDLE from one merged channel wrongly idled the whole channel.
This performs the same min-watermark / any-active aggregation on the unspilling thread.

Brief change log

• [FLINK-40517] Aggregate the min watermark (held while any merged old channel is still uninitialized) and any-active watermark-status per new channel in ChannelStateFilteringHandler; NO_RESCALE and fan-out stay verbatim.

Verifying this change

This change added tests:

• GateFilterHandlerTest cases for fan-in min-watermark merge (with hold-until-all) and any-active watermark-status; both fail on verbatim pass-through.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no (recovery-time unspilling only)
• Anything that affects deployment or recovery: yes (unaligned-checkpoint restore with in-flight channel state on rescale)
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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CI report:

• 048bc51 Azure: SUCCESS

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