Improve error message for invalid design matrix cells

[ajaust]

Issue
Resolves #14108

Approach

Errors for empty or invalid design matrix cells reported a row index into
the already-filtered dataframe, so "Row 1" rarely matched the row the user
had to fix. Rows are now read without dropping blanks, filtered in-process,
and the originating spreadsheet row is carried into the message.

Both sheets anchor their read with skip_rows, which stops the reader from
trimming blank rows above the data; without it the numbers drift by the
number of leading blanks. A parametrised test pins this.

The message also no longer claims a cell is empty when it holds a rejected
word such as NONE, and the accepted values are documented.

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When screenshots are changed: Review screenshot-PR in ert-testdata,
  merge screenshot-PR in ert-testdata before merging this PR.
• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[Copilot]

Pull request overview

This pull request improves DESIGN_MATRIX validation feedback by reporting spreadsheet row numbers (rather than indices after internal filtering) and by clarifying that failures can be due to either empty cells or explicitly rejected cell values. It also documents the rejected values in the user-facing configuration reference and updates/extends unit tests to pin the new behavior (including handling of blank rows).

Changes:

• Track original Excel row numbers while reading both design and default sheets, and use them in validation error messages.
• Treat disallowed textual values (NONE/NULL/NAN, case-insensitive, whitespace-trimmed) as invalid and reflect that in the error wording.
• Add/update documentation and unit tests to cover the new error messaging and row-number stability (including blank/leading blank rows).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/ert/unit_tests/sensitivity_analysis/test_design_matrix.py	Updates expected error messages/row numbers and adds regression tests ensuring blank rows and leading blanks don’t shift reported rows.
src/ert/config/design_matrix.py	Preserves spreadsheet row provenance via _drop_empty_rows, rewords validation errors, and centralizes disallowed cell values.
docs/ert/reference/configuration/keywords.rst	Documents which design matrix cell values are considered invalid.
docs/ert/getting_started/howto/design_matrix.rst	Updates the DESIGN_MATRIX validation description and links to the documented invalid cell values note.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.93%. Comparing base (31248f9) to head (33d9cd0).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14123      +/-   ##
==========================================
+ Coverage   91.85%   91.93%   +0.07%     
==========================================
  Files         484      482       -2     
  Lines       33581    33503      -78     
==========================================
- Hits        30847    30800      -47     
+ Misses       2734     2703      -31     

Flag	Coverage Δ
cli-tests	36.25% <78.57%> (+0.07%)	⬆️
fuzz	44.22% <21.42%> (+0.13%)	⬆️
gui-tests	58.62% <78.57%> (+0.22%)	⬆️
performance-and-unit-tests	80.81% <100.00%> (+0.34%)	⬆️
test	46.04% <21.42%> (-0.28%)	⬇️

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

Files with missing lines	Coverage Δ
src/ert/config/design_matrix.py	97.51% <100.00%> (+0.13%)	⬆️

... and 33 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

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_{Comparing ajaust:improve-designmatrix-errors (33d9cd0) with main (40ae884)}