Reduce R CMD check runtime overhead

[egillax]

Summary

• avoid storing executable tuning metric/generator closures in database hyperparameter JSON
• preserve tuning metric metadata such as name, maximize, and funArgs for custom metrics
• limit macOS coverage installation/reporting to primary branch PRs/pushes

Verification

• Rscript -e 'Sys.setenv(NOT_CRAN = "true"); devtools::test(filter = "UploadToDatabase", reporter = "summary")'\n- Rscript -e 'Sys.setenv(NOT_CRAN = "true"); devtools::test(filter = "multiplePlp", reporter = testthat::SlowReporter$new(min_time = 0.05))'\n- ruby -e 'require "yaml"; YAML.load_file(".github/workflows/R_CMD_check_Hades.yaml"); puts "yaml ok"'\n

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.76%. Comparing base (aee14bf) to head (443e335).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #669      +/-   ##
===========================================
+ Coverage    89.73%   89.76%   +0.02%     
===========================================
  Files           49       49              
  Lines        13691    13727      +36     
===========================================
+ Hits         12286    12322      +36     
  Misses        1405     1405              

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