test: add ESM plugin tests

[jescalada]

Extends plugin tests by adding fixtures for ESM plugins and testing whether they're compatible with the PluginLoader. Builds on top of #1202.

[netlify]

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Name	Link
🔨 Latest commit	6e920e3
🔍 Latest deploy log	https://app.netlify.com/projects/endearing-brigadeiros-63f9d0/deploys/6904b2bb0cb4df0008c5825e

[codecov]

Codecov Report

❌ Patch coverage is 67.92453% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.97%. Comparing base (a0e5bcc) to head (6e920e3).

Files with missing lines	Patch %	Lines
src/service/routes/push.ts	14.28%	6 Missing ⚠️
src/proxy/index.ts	50.00%	5 Missing ⚠️
src/config/index.ts	33.33%	4 Missing ⚠️
...roxy/processors/push-action/checkCommitMessages.ts	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1265      +/-   ##
==========================================
- Coverage   83.36%   80.97%   -2.39%     
==========================================
  Files          70       67       -3     
  Lines        3005     4525    +1520     
  Branches      499      773     +274     
==========================================
+ Hits         2505     3664    +1159     
- Misses        397      846     +449     
+ Partials      103       15      -88     

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[jescalada]

Closing as it'd be cleaner to update the Vitest refactor PR later!