Ensure SummaryConfig cannot be created with no keys

[eivindjahren]

Resolves #7046

• Ensures good behavior regarding setting ECLBASE but not using it.
• Document behavior of existence check in local_ensemble
• Updates and refactors stateful storage test to reflect this behavior

• 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 tests pass locally (after every commit!)

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: Create Backport PR to latest release

[frode-aarstad]

Looks good!

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5344f25) 84.71% compared to head (0037825) 85.41%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
src/ert/config/observations.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7150      +/-   ##
==========================================
+ Coverage   84.71%   85.41%   +0.69%     
==========================================
  Files         380      380              
  Lines       22633    22631       -2     
  Branches      931      936       +5     
==========================================
+ Hits        19174    19330     +156     
+ Misses       3340     3183     -157     
+ Partials      119      118       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[eivindjahren]

@frode-aarstad This cannot be merged currently though as it would break with current behavior that
summary keys from HISTORY_OBSERVATION are added to the summary config if missing.

ert/src/ert/config/observations.py

Lines 110 to 111 in 1ff65a5

	if summary_key not in response_config.keys:
	response_config.keys.append(summary_key)