feat: sanitize CLOUDINARY_URL= prefix and add inline format hint in w…

[Udilh]

…izard

• Add Connect::sanitize_connection_url() — strips the CLOUDINARY_URL= prefix (case-insensitive) and trims surrounding whitespace, covering the common copy-paste pattern from the Cloudinary dashboard.
• Wire the new method into verify_connection(), rest_test_connection(), upgrade_connection(), and maybe_connection_string_constant(), replacing the previous case-sensitive str_replace() calls.
• Wizard PHP: add #connection-format-hint

  beneath the error indicator so users see the expected URL format inline without leaving the page.

• Wizard JS: expose formatHint in the connection object; show the hint in showError(), hide it in showSuccess() and on each new keystroke; upgrade the prefix-strip regex from literal string to /^CLOUDINARY_URL=/i with a .trim() so leading/trailing whitespace is also handled client-side.
• Add PHPUnit 9 test suite (tests/php/test-class-connect.php) covering sanitize_connection_url() (8 cases) and CLOUDINARY_VARIABLE_REGEX (6 cases), plus 3 sanitize-then-validate integration tests. Bootstrap stubs the minimal WP functions so tests run without a WP installation.
• Add phpunit/phpunit ^9.6 to composer require-dev and create phpunit.xml.dist.

Fixes #123

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