chore: Remove deprecated rspec matcher dependencies

[jgraichen]

Instead, use plain Ruby code and builtin RSpec matchers to check collection sizes and other properties.

This reduces maintenance effort and dependency count.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.16%. Comparing base (8fe4dc0) to head (2dc7f83).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #103   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files          47       47           
  Lines        1158     1158           
=======================================
  Hits         1102     1102           
  Misses         56       56           

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