docs(dev): add crypto APIs, unserialize and CSV injection to security.rst - #15710

Summary

Updates developer_manual/digging_deeper/security.rst per #15681:

• Replace the Trusted-domain \OC::$server->get() example with constructor injection of ITrustedDomainHelper.
• Document short DI examples for OCP\Security\ICrypto, ISecureRandom, and IHasher.
• Warn against unserialize() on user-influenced data; prefer JSON.
• Document CSV/spreadsheet formula-injection neutralization for cells starting with =, +, -, or @.

Closes #15681

Test plan

• Sphinx build of the developer manual succeeds for the security page
• Trusted-domain example uses constructor DI (no \OC::$server)
• ICrypto / ISecureRandom / IHasher each have a short injectable example
• Deserialization section says never unserialize user data; use JSON
• CSV section documents neutralizing =, +, -, @ cell prefixes

AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.