Refactor: separate SplitAndSizeRestriction from FnApiDoFnRunner

[kennknowles]

This builds on #34883 and #34907 which are steps 1 and 2 of untangling FnApiDoFnRunner. This splits off the still-very-simple transform that calls SplitRestriction on each restriction and SizeRestriction on each of the sub-parts that is emitted.

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[scwhittle]

LGTM, with some small cleanups

[scwhittle]

nit: move down so grouped with other tests?

[kennknowles]

Done

[kennknowles]

Thanks for bearing with me and paying close attention to this! I know the diffs are not particularly helpful in seeing what the real change is. I do think the positive delta is almost all boilerplate and things are shrinking in spirit and cyclomatic complexity is reducing. Just a couple more and it'll be more like straight line code based on the URN, which should lend itself to (1) pulling out more shared and meaningful abstractions and (2) optimizing and further simplifying specific paths by finding more stuff that was in there just because it was hard to tell which operation we were performing.

[scwhittle]

Thanks for bearing with me and paying close attention to this!

No problem, thanks for making this more maintainable. I think that the fact that I added the currentTrackerClaimed creation to this method when it was part of the huge file shows how difficult it was to see what exactly was going on there :)