What is the Guardrails pattern in Spring AI? How do you implement input/output validation?

Input guardrails check user input before it reaches the LLM (prompt injection, PII, off-topic queries); output guardrails check the LLM's response (hallucination, PII leakage, policy violations). Implement both as custom CallAdvisors — pre-advisors for input, post-advisors for output — using either an LLM-based moderation call or regex/NLP checks.