Create a pattern monitor for a gate

Use when: caller wants to monitor traffic against a specific pattern (e.g. prompt-injection signature, regression detection) for a gate

Attaches a pattern monitor to a gate. Monitors continuously scan responses (or request payloads) for matching patterns — keyword, regex, semantic, or metric — and fire alerts when matches accumulate above their configured threshold. Useful for catching prompt-injection attempts, content-policy violations, or quality regressions in production.