Add more tests of Sipnet segmented restart

[infotroph]

Suggestions from @divine7022 in #4008 (comment):

1 ) ryt now test only hits CSV path in segment_dataframe, but event_json fallback case is still not covered
so could be a second test_that block that omits crop_changes and provides an event_json json ?

2 ) zit worth having a test to varify RESTART_IN / RESTART_OUT chain inside the per segment sipnet.in files ? since the chain
is whole point of segmented restarts, tiny check that segment 2's sipnet.in references segment 1's restart.out (and so on)
could help lock in the core behaviour

[codewithchotu]

Hi! @infotroph I'm a first-year CS student and an open-source contributor currently participating in GSSoC 2026. I'm interested in contributing to PEcAn and would like to work on this issue.

I am currently setting up the development environment and reviewing the relevant code and tests. If this issue is available and considered suitable for a newer contributor, I'd be happy to work on it and submit a PR.

Any guidance on where to start would be greatly appreciated. Thank you!