Tests: Add unit test coverage for WP_MatchesMapRegex

[huzaifaalmesbah]

Description

Adds a test class for WP_MatchesMapRegex, which maps $matches[] placeholders in rewrite rule queries to their matched values and had no tests. The tests cover single, repeated, out-of-order and two-digit placeholders, URL-encoding of the matched values (including multibyte characters), missing, empty and '0' matches, and the placeholders that must be left untouched: $matches[0], a leading zero, non-numeric keys and other variable names. They also cover the output property and callback() directly.

This is a test-only change; no production code is modified.

Testing Instructions

1. Run the tests on single site:
   npm run test:php -- --filter 'Tests_Rewrite_wpMatchesMapRegex'
2. Run them on multisite:
   npm run test:php -- --filter 'Tests_Rewrite_wpMatchesMapRegex' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'Tests_Rewrite_wpMatchesMapRegex' --order-by=random
4. Run the whole group to confirm nothing else is affected:
   npm run test:php -- --group rewrite
5. Run PHPCS on the test file:
   node ./tools/local-env/scripts/docker.js run --rm php ./vendor/bin/phpcs tests/phpunit/tests/rewrite/wpMatchesMapRegex.php

Trac ticket: https://core.trac.wordpress.org/ticket/65819

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Brainstorming edge-case scenarios and drafting test boilerplate; test architecture, assertions, and local PHPUnit verification were written and validated by me.

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