fix: no re-emit unhandledRejection for non-Mocha registred listener

[mostafaNazari702]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: unhandledRejection events are re-emitted #4743
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Current unhandledRejection handler used to remove itself and re-emit the event so node could apply its default behavior but this made user listeners to run twice when present.
this change skips the re-emit if any other unhandledRejection listener is registered thus preventing duplicates and in the same time keeping the existing no-listener behavior unchanged.

An integration test ensures user listeners are called exactly once.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.89%. Comparing base (6695fba) to head (c196dca).

Files with missing lines	Patch %	Lines
lib/runner.js	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5987      +/-   ##
==========================================
- Coverage   80.89%   80.89%   -0.01%     
==========================================
  Files          64       64              
  Lines        4602     4607       +5     
  Branches      976      976              
==========================================
+ Hits         3723     3727       +4     
- Misses        879      880       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mostafaNazari702]

Added only a space in the end of the PR title to re-run tests.