fix: serialize config unit tests to prevent CI race

[miguelgila]

Summary

• Fix test parallelism race in config::tests that caused CI coverage failures
• The config tests mutate the shared REAPER_CONFIG env var; without serialization, parallel test threads race on set_var/remove_var, causing intermittent NotPresent panics
• Add #[serial] attribute (from serial_test crate, already a dev-dependency) to all 4 config tests

Test plan

• cargo test config::tests — 8/8 pass (4 per binary)
• cargo clippy --all-targets — clean

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.94%. Comparing base (eaa34f4) to head (246b5a6).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   86.20%   85.94%   -0.27%     
==========================================
  Files           4        5       +1     
  Lines         174      185      +11     
==========================================
+ Hits          150      159       +9     
- Misses         24       26       +2     

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