test: fix failed ValidationTest

[kenjis]

Description

Error: tests/system/Validation/ValidationTest.php:800:53: UndefinedClass: Class, interface or enum named CodeIgniter\Validation\URI does not exist (see https://psalm.dev/019)

https://github.com/codeigniter4/CodeIgniter4/actions/runs/6916967990/job/18817589705

1) CodeIgniter\Validation\StrictRules\ValidationTest::testJsonInputInvalid
Failed asserting that exception of type "Error" matches expected exception "CodeIgniter\HTTP\Exceptions\HTTPException". Message was: "Class "CodeIgniter\Validation\URI" not found" at
/home/runner/work/CodeIgniter4/CodeIgniter4/tests/system/Validation/ValidationTest.php:800
.

https://github.com/codeigniter4/CodeIgniter4/actions/runs/6916968004/job/18817590692
Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide

[kenjis]

Now GitHub Action checks fail.
Review this first.