🐛 Bug: unexpected config properties have undefined behavior

[Vessel9817]

Bug Report Checklist

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• This is NOT a security, npm audit, or GitHub Advisory issue.
• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I have 'smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, my usage of Mocha, or Mocha itself.
• I want to provide a PR to resolve this

Expected

Without tests:

Error: No test files found: "test"

With tests:

 1 passing (0ms)

Actual

Without tests:

Error: No test files found: "https://json.schemastore.org/mocharc.json"

With tests:

Warning: Cannot find any files matching pattern "https://json.schemastore.org/mocharc.json"
 1 passing (0ms)

Overall, the $schema property should be ignored, as it's purely for providing IDE hints. Furthermore, it would be wise to pass a warning for other properties the config parser isn't aware of, before ignoring them. For instance, deprecated properties which have been removed.

Minimal, Complete and Verifiable Example

Without tests: Vessel9817/mocha-mre-1
With tests: Vessel9817/mocha-mre-1#with-tests

Versions

> mocha --version
12.0.0-rc.6
> &"./node_modules/.bin/mocha" --version
12.0.0-rc.6
> node --version
v24.16.0

• Environment: Node
• Shell: PowerShell
• Other modules: cross-env (optional, for using debug)

Additional Info

• For convenience, DEBUG=mocha:* is set.
• The other branches Vessel9817/mocha-mre-1#success and Vessel9817/mocha-mre-1#success-with-tests show the same MCVEs, but with Mocha v11. This is to show that the issue is specific to the pre-release of Mocha version 12.