refactor(index): enforce strict regex

[Fdawgs]

u unicode flag is equivalent to 'use strict' directive in JS

Checklist

• run npm run test && npm run benchmark --if-present
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Copilot]

Pull request overview

This PR adds the u (unicode) flag to four regular expression patterns in the Basic Auth plugin to enforce stricter and more predictable regex behavior.

Key changes:

• Added u flag to credentialsStrictRE and credentialsLaxRE (combining with existing i flag)
• Added u flag to controlRE and userPassRE patterns

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