test: Increase Email test coverage and enable component in random tests

[gr8man]

Description
Ref: #9968

• Added additional test cases for the Email class (covering header injection prevention, SMTP disconnection handling, UTF-8 filenames, and other edge cases).
• Resolved global state pollution issues in namespace overrides by implementing cleanup in tearDown().
• Enabled the Email component in the random tests execution configuration (random-tests-config.txt).
• Fixed a PHPDoc type typo for the $newline property in Email.php.

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]

Please remove the newly added tests that depend on the namespace-level native-function mocks, together with the global variables used to control them.

Although namespace-level function shadowing can be useful for isolated cases, using it here across nine native functions creates persistent process-level behavior and substantial global test state. This makes the tests difficult to isolate and maintain, and several mocks replace native behavior even when a test has not explicitly configured them.

[michalsn]

You're weakening annotations. Please revert.

[michalsn]

If this method is needed, you have to call parent::tearDown().

[gr8man]

I cannot fix the PHPDoc type typo (@var "\r\n"|"n") directly in system/Email/Email.php because of an upstream issue with PHP-CS-Fixer's global_namespace_import rule.

Since Email.php has a namespace (CodeIgniter\Email), the fixer incorrectly treats the \n inside the PHPDoc string as a global class, stripping the backslash and erroneously injecting a use n; statement at the top of the file, which breaks the build.