[hotfix][checkpoint] Refactor common behavior into ChannelStateFilteringHandler#getGateFilterHandler and GateFilterHandler#getVirtualChannelBy to reduce code duplication

[RocMarshal]

What is the purpose of the change

[hotfix][checkpoint] Refactor common behavior into ChannelStateFilteringHandler#getGateFilterHandler and GateFilterHandler#getVirtualChannelBy to reduce code duplication in ChannelStateFilteringHandler.java

Brief change log

[hotfix][checkpoint] Refactor common behavior into ChannelStateFilteringHandler#getGateFilterHandler and GateFilterHandler#getVirtualChannelBy to reduce code duplication in ChannelStateFilteringHandler.java

Verifying this change

This change is already covered by existing tests

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CI report:

• 90ec280 Azure: SUCCESS

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[weiqingy]

Thanks for the cleanup. I diffed each new helper against both copies of the code it came from and the bodies are identical, so nothing changed in the move. One small note inline.

[weiqingy]

nit: The comment at line 363 marks everything below it as code that goes away when FLINK-38544 lands. This new helper sits inside that section, but one of its two callers is at line 325, in the overload that stays. Up to now nothing inside the section was called from outside it, so that deletion could be a clean cut. Would moving the helper above line 362 be worth it to keep that true?