[Bug][Go SDK]: Checkpointing should "split" restrictions to claimed positions.

[lostluck]

What happened?

The Go SDK doesn't currently handle process continuations correctly when partial work has been done.

The correct behavior is that any claimed positions are assumed to be processed by the primary, leaving the rest to the residual. That is, it should be possible for a DoFn to eventually process all of a restriction a single position at a time, by always returning process continuation delays after each claimed restriction.

At present, the original restriction is what is used, instead splitting the restriction at the claimed position boundary, leading to infinite processing.

Related #24931 which rendered these independent by element.

Issue Priority

Priority: 1 (data loss / total loss of function)

Issue Components

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[lostluck]

Further validation showed that this is already how the SDK works. I must have remembered some older bad output from the replacement runner for #24789.

I may circle back to this with some additional "split on 0" and "split on 1" unit tests for the offset range restriction, but it's currently a non-issue.