test: add unit tests for GDHandler save and security exceptions

[gr8man]

Description
Adds missing unit tests for GDHandler::save() and unsupported image types exceptions to improve code coverage.

Changes:

• Added tests for file save failures using vfsStream.
• Added a test for the save($target, 100) unmodified copy logic.
• Added tests simulating an unsupported imageType to ensure ImageException is thrown instead of a fatal GD error.

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