Use context manager and match-case in endpoints

[berland]

Issue
Readability.

Approach
contextlib.

• 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 63.35878% with 48 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.24%. Comparing base (005c621) to head (ee22a67).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/ert/dark_storage/endpoints/responses.py	54.38%	26 Missing ⚠️
src/ert/dark_storage/common.py	40.00%	12 Missing ⚠️
src/ert/dark_storage/endpoints/parameters.py	62.50%	9 Missing ⚠️
src/ert/dark_storage/endpoints/experiments.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12934      +/-   ##
==========================================
+ Coverage   90.12%   90.24%   +0.12%     
==========================================
  Files         445      446       +1     
  Lines       30858    30820      -38     
==========================================
+ Hits        27811    27815       +4     
+ Misses       3047     3005      -42     

Flag	Coverage Δ
cli-tests	36.89% <0.00%> (+0.05%)	⬆️
gui-tests	67.44% <3.81%> (+0.07%)	⬆️
performance-and-unit-tests	76.85% <63.35%> (+0.10%)	⬆️
test	46.08% <24.42%> (+0.07%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 35 untouched benchmarks

---

_{Comparing berland:http_errors_ctx (c355e3d) with main (0790612)}

[berland]

My sneaky force-push also moved two lines in get_gradient into the reraise.. context, consistent with the other code.

[andreas-el]

Well done! 🥓