chore: replace Karma with Playwright for browser tests

[mostafaNazari702]

PR Checklist

• Addresses an existing open issue: fixes 🛠️ Repo: Stop using deprecated karma library #5253
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

This is from a very old issue but i replaced the deprecated Karma setup with Playwright which ships its own bundled Chromium (no system Chrome required). The four browser suites (bdd/tdd/qunit/esm) now run in a real Chromium page via a small static server that serves the built mocha.js and a generated per-suite runner with results asserted via @playwright/test. npm run test-browser keeps the same clean=>build=>browser=>webpack-compat flow and CI installs the browser with npx playwright install --with-deps chromium.

[mostafaNazari702]

@mark-wiemer You did show some passion in opening a PR for that issue but i had the same passion. Sorry :)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.88%. Comparing base (a8ffaa9) to head (ceb86af).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
playwright.config.mjs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6031      +/-   ##
==========================================
+ Coverage   78.08%   78.88%   +0.79%     
==========================================
  Files          63       63              
  Lines        4581     4546      -35     
  Branches      978      969       -9     
==========================================
+ Hits         3577     3586       +9     
+ Misses       1004      960      -44     

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[mark-wiemer]

Test failures are known issues, ref #5361

This one is on the review backlog :) it's a big backlog though!

And absolutely no problem that you opened a PR for this, I'm often much happier reviewing PRs than I am trying to write code myself :)

[JoshuaKGoldberg]

Overall LGTM! This is really exciting, karma's been a pain for a while. Just requesting changes on trimming down the large number of IMO unnecessary comments. Could you please do a pass and trim them down to what you think is useful long-term?

[mark-wiemer]

As Mostafa is now without laptop for a while, I'll validate and make the proposed changes and send back to you for re-review :)

[mark-wiemer]

hm, checking lint failure, can't repro on local quite yet...

/home/runner/work/mocha/mocha/docs/_data/supporters.cjs
Error:    20:23  error  /home/runner/work/mocha/mocha/docs/node_modules/debug/package.json (directory description file): SyntaxError: /home/runner/work/mocha/mocha/docs/node_modules/debug/package.json (directory description file): SyntaxError: /home/runner/work/mocha/mocha/docs/node_modules/debug/package.json (directory description file): SyntaxError: Unexpected end of JSON input                                                                                                                n/no-missing-require
Error:   305:11  error  /home/runner/work/mocha/mocha/docs/node_modules/debug/package.json (directory description file): SyntaxError: /home/runner/work/mocha/mocha/docs/node_modules/debug/package.json (directory description file): SyntaxError: /home/runner/work/mocha/mocha/docs/node_modules/debug/package.json (directory description file): SyntaxError: /home/runner/work/mocha/mocha/docs/node_modules/debug/package.json (directory description file): SyntaxError: Unexpected end of JSON input  n/no-missing-require

I think this was a transient thing, I can't repro with the same Node version (22.22.3) on Windows :/ going to retrigger once the rest of the workflow finished

[mark-wiemer]

Transient lint failure resolved, not going to track it... yet... but I'll link #5361 for funsies

[JoshuaKGoldberg]

transient lint failure

Yeah, it is - it's a failure to fully download the package.json file in node_modules. That's on the GHA side.

[JoshuaKGoldberg]

LGTM! Ready to ship whenever you are @mark-wiemer 🥳