[Fix] Skip parallel plot tests without multicore

[antoniojbt]

What was changed and why

• Added skip guard for multicore-dependent plot tests so they don't run when multicore futures aren't supported or CRAN core limits are active.

Tests added

• tests/testthat/test-plot-parallel.R

Backward compatibility

• No breaking changes.

Code coverage change

• No change expected.

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https://chatgpt.com/codex/tasks/task_e_68b205a2452c83268793cca961df0c82

[Copilot]

Pull Request Overview

This PR adds proper test environment handling for parallel plotting functionality by skipping multicore-dependent tests when the environment doesn't support multicore processing or when CRAN imposes core limitations.

• Introduces conditional test skipping for parallel plotting tests
• Prevents false test failures on systems without multicore support
• Adds placeholder test structure for future parallel plotting functionality

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.97%. Comparing base (e412fcb) to head (98150cf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #143   +/-   ##
=======================================
  Coverage   83.97%   83.97%           
=======================================
  Files          64       64           
  Lines        1423     1423           
=======================================
  Hits         1195     1195           
  Misses        228      228           

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