Filter NaN parameter rows in EnIF update

[larsevj]

Surface and field parameters commonly have NaN for inactive grid cells. When EnIF stacks all parameters into a joint matrix, these NaN values corrupt boost_linear_regression (np.argmax treats NaN as maximum) and propagate through StandardScaler, contaminating whichever parameter sits at index 0 in the joint matrix.

Issue
Resolves #my_issue

Approach
Short description of the approach

(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 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')

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.87097% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.24%. Comparing base (58cc8d7) to head (a38553e).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/analysis/_enif_update.py	83.87%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13202      +/-   ##
==========================================
- Coverage   90.26%   90.24%   -0.03%     
==========================================
  Files         453      453              
  Lines       31301    31327      +26     
==========================================
+ Hits        28255    28272      +17     
- Misses       3046     3055       +9     

Flag	Coverage Δ
cli-tests	37.43% <48.38%> (+<0.01%)	⬆️
gui-tests	67.38% <48.38%> (-0.04%)	⬇️
performance-and-unit-tests	77.78% <83.87%> (+<0.01%)	⬆️
test	46.33% <3.22%> (-0.04%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

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_{Comparing larsevj:enif_nan_filtering (a38553e) with main (30605cb)}