Perf: Limit mapping count sleep to 4store

[mdorf]

Summary

• skip the fixed five-second per-ontology delay on non-4store backends
• preserve the existing delay and health check on 4store
• add regression coverage for both backend paths

Implements #305

Impact

Non-4store mapping-count runs no longer incur 5 × ontology count seconds of unnecessary delay. Existing 4store pacing remains unchanged.

Verification

• bundle exec ruby -Itest test/models/test_mapping_counts_sleep.rb — 2 runs, 6 assertions
• bundle exec ruby -Itest test/models/test_mappings.rb — 5 runs, 196 assertions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.85%. Comparing base (139c67e) to head (5858742).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #308      +/-   ##
===========================================
+ Coverage    81.83%   81.85%   +0.01%     
===========================================
  Files          101      101              
  Lines         6849     6849              
===========================================
+ Hits          5605     5606       +1     
+ Misses        1244     1243       -1     

Flag	Coverage Δ
ag	81.76% <100.00%> (+0.02%)	⬆️
fs	81.79% <100.00%> (+0.04%)	⬆️
gd	81.74% <100.00%> (-0.03%)	⬇️
unittests	81.85% <100.00%> (+0.01%)	⬆️
vo	81.74% <100.00%> (+0.01%)	⬆️

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