test(sipnet): cover JSON segmentation and restart chaining

[ANAMASGARD]

Description

• Added a regression test for segment_dataframe() using event_json when crop_changes is absent. Verify site filtering, segment dates and IDs, crop codes, and the initial PFT.
• Verify restart assignments in generated sipnet.in files: the first segment has no RESTART_IN, each segment writes its own RESTART_OUT, and subsequent segments read the previous segment’s output.
• Update SIPNET NEWS and the root changelog.

No production code or dependencies are changed.

Motivation and Context

Closes #4021.

Adds the two coverage improvements suggested during review of #4008, protecting JSON-based segmentation and the restart chain without requiring a SIPNET executable.

Validation

• Focused segmented-config tests: 25 assertions passed.
• Full SIPNET test suite: 132 assertions passed.
• Package check with RUN_TESTS=TRUE: failed because existing tests use PEcAn.settings without declaring it as a test dependency. The same failure was reproduced on an unchanged baseline copy.

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The package-check failure remains unresolved, so the overall test checklist below is left unchecked.

Review Time Estimate

• When possible

Types of changes

Test coverage only; no runtime behavior changes.

Checklist:

• My change requires a change to the documentation.
• My name is in the list of CITATION.cff
• I agree that PEcAn Project may distribute my contribution under any or all of
  • the same license as the existing code,
  • and/or the BSD 3-clause license.
• I have updated the CHANGELOG.md.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Check the license agreement yourself before submitting, and the citation checkbox only if your name is already listed.

[infotroph]

Thanks! This looks good except for one place I may have moved the ground under you -- can you confirm this test still behaves right after #4110?