Replace OpenSSL with native CSPRNG for token generation

[yann-eugone]

This PR deprecates the usage of openssl_random_pseudo_bytes in favor of the native random_bytes function.

Key Changes:

1. Security Standard Upgrade (CSPRNG): Switched to random_bytes, which is currently considered the industry state-of-the-art for generating cryptographic entropy in PHP. Unlike OpenSSL, it is native to the language and provides a fail-closed mechanism (throws an Exception instead of returning weak data if entropy is insufficient).

2. Length Accuracy: Implemented a calculation logic to ensure the generated hex string matches exactly the requested $length.

References & Resources:

• PHP Manual: random_bytes() - CSPRNG
• OWASP: Cryptographic Storage Cheatsheet - Rule: Use a cryptographically strong pseudo-random number generator
• Paragonie (Security Experts): Why you should use random_bytes()

Why this change is necessary
openssl_random_pseudo_bytes relies on the OpenSSL extension availability and historically required manual verification of the $crypto_strong boolean to avoid silent failures. random_bytes abstracts access to the OS's entropy source (e.g., /dev/urandom) securely and natively.