build: declare PHPUnit and Polyfills as composer dependencies

[i-am-chitti]

Description

Currently, running tests locally via npm run test fails for local developers with a fatal error on a fresh install:

Error: The PHPUnit Polyfills library is a requirement for running the WP test suite.
The PHPUnit Polyfills autoload file was not found in "/var/www/html/vendor/yoast/phpunit-polyfills"

This occurs because:

1. yoast/phpunit-polyfills and phpunit are not declared in composer.json.
2. The core tests container setup expects these to be in /var/www/html/vendor/, which is only installed on-the-fly during CI runs.

This PR cleans up the local development and CI testing environments by:

• Adding phpunit/phpunit and yoast/phpunit-polyfills as dev dependencies inside composer.json.
• Pointing .wp-env.json to look inside the mounted plugin's vendor folder for the polyfills.
• Updating package.json to execute ./vendor/bin/phpunit (PHPUnit 9) instead of the WordPress core vendor path. wp-env starts with a standard WordPress ZIP download by default, which does not contain a vendor/ directory inside /var/www/html .
• Aligning both GitHub Action workflows (phpunit.yml and multisite.yml) to run a standard composer install --no-interaction and run ./vendor/bin/phpunit. This removes the slow and redundant container-level composer require run.

Fixes

• Local PHPUnit tests fail to boot via wp-env due to missing PHPUnit Polyfills #148

How to Test / Verification

1. Install dependencies:

   composer install

2. Execute the tests:

   npm run test

3. Verify that the tests boot correctly and all unit tests run and pass.

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity
Model(s): Gemini 3.5 Flash
Used for: Identifying the local test suite provisioning failure, and find regression issue like adjusting the GitHub Action workflows to use standard composer install. Each change has been humanly reviewed by me.

[github-actions]

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Props iamchitti.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.56%. Comparing base (3a2ae99) to head (07e43e0).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #150   +/-   ##
=========================================
  Coverage     28.56%   28.56%           
  Complexity      161      161           
=========================================
  Files            15       15           
  Lines          1817     1817           
=========================================
  Hits            519      519           
  Misses         1298     1298           

Flag	Coverage Δ
phpunit	28.56% <ø> (ø)

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