[Fix] Use multisession plan in parallel plotting test

[antoniojbt]

What was changed and why

• Add tests/testthat/test-plot-parallel.R to exercise parallel plotting utilities using a cross-platform multisession plan.
• Ensure each test tears down workers immediately with future::plan("sequential") via withr::defer.

Tests added

• tests/testthat/test-plot-parallel.R

Backward compatibility

• No breaking changes.

Code coverage change

• No significant change expected.

---

https://chatgpt.com/codex/tasks/task_e_68b20598d1b8832691f8251e9349f18d

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.24%. Comparing base (b7e156c) to head (bb1cf03).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   86.51%   87.24%   +0.73%     
==========================================
  Files          65       66       +1     
  Lines        1379     1435      +56     
==========================================
+ Hits         1193     1252      +59     
+ Misses        186      183       -3     

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[Copilot]

Pull Request Overview

This PR adds a new test file to exercise parallel plotting utilities using the multisession plan for cross-platform compatibility. The test ensures proper worker teardown and validates the multicore functionality in plotting contexts.

Key changes:

• Adds comprehensive parallel plotting test coverage using multisession plan
• Implements proper worker cleanup with withr::defer
• Ensures cross-platform compatibility by avoiding platform-specific multicore plan