Warn and skip constant parameter groups in all update strategies

[larsevj]

Issue
Resolves #13695

• 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 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.56%. Comparing base (cb1a099) to head (a0796af).

Files with missing lines	Patch %	Lines
src/ert/analysis/_update_strategies/_adaptive.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13694      +/-   ##
==========================================
- Coverage   89.58%   89.56%   -0.02%     
==========================================
  Files         469      469              
  Lines       33419    33427       +8     
==========================================
+ Hits        29938    29939       +1     
- Misses       3481     3488       +7     

Flag	Coverage Δ
cli-tests	35.77% <25.00%> (-0.01%)	⬇️
fuzz	43.36% <12.50%> (-0.01%)	⬇️
gui-tests	59.25% <25.00%> (-0.01%)	⬇️
performance-and-unit-tests	77.79% <87.50%> (-0.01%)	⬇️
test	45.53% <0.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
src/ert/analysis/_es_update.py	93.98% <100.00%> (+0.28%)	⬆️
src/ert/analysis/_update_strategies/_adaptive.py	96.00% <50.00%> (-1.92%)	⬇️

... and 2 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

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_{Comparing larsevj:handle_constant_params_in_adaptive_localizations (a0796af) with main (cb1a099)}

[Copilot]

Pull request overview

This PR addresses failures in ensemble updates when a parameter group is constant (zero variance across realizations) by explicitly detecting that condition, warning, and skipping the update while carrying the parameters over unchanged. This fits into ERT’s analysis update pipeline (_es_update + per-strategy implementations) by preventing update strategies from being invoked with empty/invalid parameter selections.

Changes:

• Add an early-exit in perform_ensemble_update to warn, emit a status event, and copy parameters unchanged when an entire parameter group has zero variance.
• Harden the adaptive localization strategy to skip assimilation for batches where all parameters are zero-variance (empty update set).
• Add a regression unit test asserting a warning is logged and the posterior parameters equal the prior for constant parameters (for both global and adaptive strategies on GenKwConfig).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/ert/unit_tests/analysis/test_es_update.py	Adds regression test ensuring constant parameter groups are warned about and carried over unchanged (adaptive/global for GenKw).
src/ert/analysis/_update_strategies/_adaptive.py	Skips adaptive assimilation for batches where zero-variance filtering yields an empty update set.
src/ert/analysis/_es_update.py	Warns and skips updates when all parameters in a group have zero variance; carries them over unchanged to the posterior.

[berland]

Looks good

[scout-team-app]

Successfully created backport PR for version-22.0:

• [Backport version-22.0] Warn and skip constant parameter groups in all update strategies #13709