Fixed DEBUG variable effects in the vulnerable server code.

[tanaydin]

With this change, it's checking for VULN_SERVER_DEBUG environment variable and sets to True if it's in the list of some common True values as strings. If it's set to True, it's printing some information about current process in the vulnerable server.

[Imran-imtiaz48]

This code introduces a structured debug logging mechanism to the vulnserver, making the server’s internal operations much more transparent and easier to troubleshoot. By allowing debug messages to be toggled via the VULN_SERVER_DEBUG environment variable, developers and testers can selectively enable detailed logs without altering the codebase. The new debug_print function provides consistent, clearly prefixed debug output throughout the request handling, database interactions, error reporting, and server lifecycle events. This improves maintainability and traceability, making it easier to identify issues during development or security testing. Importantly, the changes maintain backward compatibility and do not impact the normal flow or performance when debugging is turned off, representing a valuable quality-of-life enhancement for anyone working with or extending the server.