CI: Add comprehensive compatibility checks and PHP syntax validation

[HurayraIIT]

Description

Adds extensive CI checks to the embedpress-compatibility-test.yml workflow, including:

• Pre-matrix Version Consistency Check (validates embedpress.php, includes.php, and readme.txt).
• Pre-matrix PHP Syntax Check (php -l) across PHP 7.4 through 8.5.
• Validation for critical constants and default options.
• Validation for shortcode registration and REST API routes.
• Verification that all 5 Analytics database tables are correctly created upon activation.
• Enhanced PHP error catching (fatals fail the build, deprecation notices emit workflow warnings).

These checks will run independently and then as part of the WP 6.0 - 6.9 compatibility matrix.

[HurayraIIT]

@akashmdiu vai, please review the PR 🙂