ENH: Add RLE support for ZCORN

[alifbe]

Resolves #1486

Add support for ZCORN keyword to Run-Length Encoding compression. This is particularly useful for horizontal grid

Checklist

• Tests added (if not, comment why)
• Test coverage equal or up from main (run pytest with --cov=src/ --cov-report term-missing)
• If not squash merging, every commit passes tests
• Appropriate commit prefix and precise commit message used
• All debug prints and unnecessary comments removed
• Docstrings are correct and updated
• Documentation is updated, if necessary
• Latest main rebased/merged into branch
• Added comments on this PR where appropriate to help reviewers
• Moved issue status on project board
• Checked the boxes in this checklist ✅

[Copilot]

Pull request overview

This PR adds Run-Length Encoding (RLE) compression support for the ZCORN keyword when exporting grid data to GRDECL format. Previously, RLE compression was only available for the ACTNUM keyword. This enhancement is particularly beneficial for horizontal grids where ZCORN values have many consecutive repetitions.

Key Changes:

• Extended RLE compression to include ZCORN in addition to ACTNUM
• Updated test validation to verify compression works correctly for both ZCORN and ACTNUM keywords
• Improved test robustness with a helper function to parse specific keyword sections

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/xtgeo/grid3d/_grdecl_grid.py	Modified the RLE condition to include "ZCORN" alongside "ACTNUM" for compression
tests/test_grid3d/test_grid_property.py	Enhanced test validation with section-specific token parsing and verification of ZCORN compression

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