refactor: prepend Autoloader's own autoload functions

[paulbalandan]

Description
Random tests fail haphazardly with cannot autoload CodeIgniter\CodeIgniter.

From this failing test details for DataCaster, it looks to me that composer's autoloader are in effect here.

Details

> vendor/bin/phpunit tests/system/DataCaster --colors=never --no-coverage --order-by=random --random-order-seed=1775670338
PHPUnit 11.5.55 by Sebastian Bergmann and contributors.
Runtime:       PHP 8.5.4

Configuration: /home/runner/work/CodeIgniter4/CodeIgniter4/phpunit.xml.dist

Random Seed:   1775670338
E                                                                   1 / 1 (100%)
Time: 00:00.011, Memory: 12.00 MB
There was 1 error:

CodeIgniter\DataCaster\DataCasterTest::testCastInvalidMethodException

ErrorException: include(/home/runner/work/CodeIgniter4/CodeIgniter4/vendor/composer/../../system/CodeIgniter.php): Failed to open stream: No such file or directory

/home/runner/work/CodeIgniter4/CodeIgniter4/system/Test/Mock/MockCodeIgniter.php:18

/home/runner/work/CodeIgniter4/CodeIgniter4/system/Test/CIUnitTestCase.php:519

/home/runner/work/CodeIgniter4/CodeIgniter4/system/Test/CIUnitTestCase.php:245
ERRORS!

Tests: 1, Assertions: 0, Errors: 1.
Execution order file: /home/runner/work/CodeIgniter4/CodeIgniter4/build/random-tests/random_test_order_176_DataCaster.txt

Failed test: CodeIgniter\DataCaster\DataCasterTest::testCastInvalidMethodException

Previous test: (none - failed test ran first)
Exit code: 1

Elapsed time: 176

I temporarily upped the run limit of random tests to 50 to see if there are still flaky tests.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

I was trying to dig into this "No such file or directory" problem, but without any luck.