fix: #3582960 validate every multiple input item

Closes #3582960

Summary

• validate every item in a multiple input instead of returning after the first item;
• preserve the emitted violations so multiple invalid values are all reported;
• pin the existing empty-array contract: required means the input is present, while minimum cardinality requires an explicit Count constraint.

Verification

• php -l src/TypedInputsTrait.php
• php -l tests/src/Kernel/Tool/TypedInputsValidationTest.php
• composer validate --no-check-publish
• git diff --check

The focused Kernel test is included but was not executed locally because this isolated worktree has no installed Drupal/PHPUnit runtime. It should run in the project's Drupal 10.5 and Drupal 11 CI jobs.

AI-assisted code: AI helped implement and review this change. The diff was independently reviewed, and its runtime test remains pending upstream CI.