Refactor MockZMQServer signal into booleans

[eivindjahren]

• 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 90.51%. Comparing base (ab16644) to head (189198a).
⚠️ Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12937      +/-   ##
==========================================
- Coverage   90.67%   90.51%   -0.16%     
==========================================
  Files         441      443       +2     
  Lines       30495    30596     +101     
==========================================
+ Hits        27650    27693      +43     
- Misses       2845     2903      +58     

Flag	Coverage Δ
cli-tests	37.11% <ø> (-0.11%)	⬇️
gui-tests	67.70% <ø> (-0.08%)	⬇️
performance-and-unit-tests	77.01% <ø> (-0.13%)	⬇️
test	46.28% <ø> (+8.99%)	⬆️

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

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

[xjules]

Some failing tests, not sure if they are related though. @jonathan-eq has made a suggestion too and thus maybe he can do a review / combine both approaches.
Ref: #12807

[codspeed-hq]

Merging this PR will improve performance by 83.27%

⚡ 1 improved benchmark
✅ 33 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_direct_dark_performance[gen_x: 20, sum_x: 20 reals: 10-gen_data_with_obs-get_record_parquet]	40.2 ms	21.9 ms	+83.27%

---

_{Comparing eivindjahren:refactor_mock_zeromq_signal (189198a) with main (c0bda0b)}

[jonathan-eq]

LGTM!