fix(cache): set secure 0755 permissions mode when creating directory in FileVarExportHandler

[gr8man]

Description
FileVarExportHandler::save() previously invoked @mkdir($this->path, 0777, true) when the cache directory did not exist. Passing 0777 grants full read/write/execute permissions to all users on systems with permissive umask settings (e.g. umask=0000 in Docker containers or shared hosting environments).

This PR changes the directory creation mode to 0755, consistent with existing CodeIgniter 4 practices (such as filesystem_helper and generator traits).

• Updated FileVarExportHandler::save() directory creation mode to 0755.
• Added regression unit test testSaveCreatesDirectoryWithCorrectPermissions() in FactoriesCacheFileVarExportHandlerTest.

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 (Not applicable - security internal fix)
• Conforms to style guide