Add extra=forbid to pydantic basemodels in ert storage and cleanup

[erlenlh]

Issue
Resolves #13520

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.95%. Comparing base (7aa972d) to head (d79859c).
⚠️ Report is 30 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13521      +/-   ##
==========================================
- Coverage   89.95%   89.95%   -0.01%     
==========================================
  Files         459      460       +1     
  Lines       32173    32409     +236     
==========================================
+ Hits        28942    29154     +212     
- Misses       3231     3255      +24     

Flag	Coverage Δ
cli-tests	37.05% <100.00%> (-0.29%)	⬇️
fuzz	44.02% <100.00%> (-0.13%)	⬇️
gui-tests	66.59% <100.00%> (-0.21%)	⬇️
performance-and-unit-tests	77.97% <100.00%> (+<0.01%)	⬆️
test	45.69% <100.00%> (-0.09%)	⬇️

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

Files with missing lines	Coverage Δ
src/ert/storage/local_ensemble.py	96.12% <ø> (+0.20%)	⬆️
src/ert/storage/local_experiment.py	96.38% <100.00%> (ø)
src/ert/storage/local_storage.py	92.70% <100.00%> (-1.05%)	⬇️

... and 29 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

---

_{Comparing ert-storage-update-basemodel (d79859c) with main (0175a5d)}

[Copilot]

Pull request overview

Adds extra="forbid" to all Pydantic BaseModel classes under src/ert/storage, ensuring that loading storage index/failure JSON rejects unknown fields rather than silently ignoring them. This aligns these internal storage models with the convention already used in Everest config models and other ERT event models.

Changes:

• _Migrations and top-level _Index in local_storage.py now forbid extra fields.
• ExperimentStatus and experiment _Index in local_experiment.py now forbid extra fields.
• Ensemble _Index and _Failure in local_ensemble.py now forbid extra fields.

Reviewed changes

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

File	Description
src/ert/storage/local_storage.py	Add extra="forbid" to _Migrations and _Index.
src/ert/storage/local_experiment.py	Add extra="forbid" to ExperimentStatus and _Index.
src/ert/storage/local_ensemble.py	Add extra="forbid" to _Index and _Failure.