fix(streaming): assert all states finished, or all not finished for backfill

[kwannoel]

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What's changed and what's your intention?

There should not be a mix of finished / unfinished states. If some partitions are unfinished, on recovery the stream job will be cleaned. But if we scale in after finishing / reschedule the actors in some way to merge partitions, perhaps the current_pos could be different.

Follow up: #12768

Checklist

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[codecov]

Codecov Report

Merging #12767 (e752660) into main (4ce28cd) will increase coverage by 0.00%.
Report is 4 commits behind head on main.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main   #12767   +/-   ##
=======================================
  Coverage   69.26%   69.26%           
=======================================
  Files        1480     1480           
  Lines      243551   243551           
=======================================
+ Hits       168700   168701    +1     
+ Misses      74851    74850    -1     

Flag	Coverage Δ
rust	69.26% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...tream/src/executor/backfill/no_shuffle_backfill.rs	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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